Upgrade minimum required Flutter version to 1.12 for our example application (#76)

Supress the following warning generated when running the example application with any flutter version > 1.12.
We are using modules that have been deprecated in Flutter SDK version 1.12.

Running the Android emulator app produces the following warning of deprecation:
```
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Warning
──────────────────────────────────────────────────────────────────────────────
Your Flutter application is created using an older version of the Android
embedding. It's being deprecated in favor of Android embedding v2. Follow the
steps at
https://flutter.dev/go/android-project-migration
to migrate your project.
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
```

The changes in this PR apply the steps outlined in https://flutter.dev/go/android-project-migration

Note: this only affects running the example. Our plugin is still backwards compatible to older versions.

Author: amit-nayar

android/config.gradle

@@ -31,7 +31,7 @@ if (pspdfkitVersion == null || pspdfkitVersion == '') {
 
 ext.pspdfkitMavenModuleName = 'pspdfkit'
 
-ext.pspdfkitFlutterVersion = '1.10.0'
+ext.pspdfkitFlutterVersion = '1.10.3'
 
 ext.androidCompileSdkVersion = 29
 ext.androidBuildToolsVersion = '29.0.1'

example/android/app/src/main/AndroidManifest.xml

@@ -15,33 +15,33 @@
     -->
     <uses-permission android:name="android.permission.INTERNET"/>
 
-    <!-- io.flutter.app.FlutterApplication is an android.app.Application that
-         calls FlutterMain.startInitialization(this); in its onCreate method.
-         In most cases you can leave this as-is, but you if you want to provide
-         additional functionality it is fine to subclass or reimplement
-         FlutterApplication and put your custom class here. -->
     <application
-        android:name="com.pspdfkit.flutter.example.FlutterExampleApplication"
+        android:name="androidx.multidex.MultiDexApplication"
         android:label="pspdfkit_example"
         android:icon="@mipmap/ic_launcher">
         <activity
-            android:name="com.pspdfkit.flutter.example.MainActivity"
+            android:name="io.flutter.embedding.android.FlutterActivity"
             android:launchMode="singleTop"
             android:theme="@style/LaunchTheme"
             android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale"
             android:hardwareAccelerated="true"
             android:windowSoftInputMode="adjustResize">
-            <!-- This keeps the window background of the activity showing
-                 until Flutter renders its first frame. It can be removed if
-                 there is no splash screen (such as the default splash screen
-                 defined in @style/LaunchTheme). -->
+            <!-- Specify that the launch screen should continue being displayed -->
+            <!-- until Flutter renders its first frame. -->
             <meta-data
-                android:name="io.flutter.app.android.SplashScreenUntilFirstFrame"
-                android:value="true" />
+                android:name="io.flutter.embedding.android.SplashScreenDrawable"
+                android:resource="@drawable/launch_background" />
+            <!-- Theme to apply as soon as Flutter begins rendering frames -->
+            <meta-data
+                android:name="io.flutter.embedding.android.NormalTheme"
+                android:resource="@style/NormalTheme" />
             <intent-filter>
                 <action android:name="android.intent.action.MAIN"/>
                 <category android:name="android.intent.category.LAUNCHER"/>
             </intent-filter>
         </activity>
+        <meta-data
+            android:name="flutterEmbedding"
+            android:value="2" />
     </application>
 </manifest>

example/android/app/src/main/java/com/pspdfkit/flutter/example/FlutterExampleApplication.java

@@ -13,4 +13,13 @@
              Flutter draws its first frame -->
         <item name="android:windowBackground">@drawable/launch_background</item>
     </style>
+    <!-- Theme applied to the Android Window as soon as the process has started.
+         This theme determines the color of the Android Window while your
+         Flutter UI initializes, as well as behind your Flutter UI while its
+         running.
+
+         This Theme is only used starting with V2 of Flutter's Android embedding. -->
+    <style name="NormalTheme" parent="@android:style/Theme.Black.NoTitleBar">
+        <item name="android:windowBackground">?android:colorBackground</item>
+    </style>
 </resources>

example/android/app/src/main/java/com/pspdfkit/flutter/example/MainActivity.java

@@ -5,6 +5,7 @@ author: PSPDFKit
 homepage: https://pspdfkit.com/
 environment:
   sdk: '>=2.0.0 <3.0.0'
+  flutter: ">=1.12.0 <2.0.0"
 
 dependencies:
   flutter:
@@ -32,7 +33,7 @@ flutter:
   # included with your application, so that you can use the icons in
   # the material Icons class.
   uses-material-design: true
-  
+
   assets:
     - PDFs/
     - PDFs/Instant/