android: Switch app ID to that of the main app

On Android, switch to using "com.zulipmobile" as the
application ID.

But keep using "com.zulip.flutter" as the JVM package name for
Java/Kotlin code.

Updates: #1582

Author: rajveermalviya

android/app/build.gradle

@@ -38,7 +38,7 @@ android {
     }
 
     defaultConfig {
-        applicationId "com.zulip.flutter"
+        applicationId "com.zulipmobile"
         minSdkVersion 28
         targetSdkVersion flutter.targetSdkVersion
         // These are synced to local.properties from pubspec.yaml by the flutter tool.

lib/model/store.dart

@@ -1078,7 +1078,7 @@ class LiveGlobalStore extends GlobalStore {
     // What directory should we use?
     //   path_provider's getApplicationSupportDirectory:
     //     on Android, -> Flutter's PathUtils.getFilesDir -> https://developer.android.com/reference/android/content/Context#getFilesDir()
-    //       -> empirically /data/data/com.zulip.flutter/files/
+    //       -> empirically /data/data/com.zulipmobile/files/
     //     on iOS, -> "Library/Application Support" via https://developer.apple.com/documentation/foundation/nssearchpathdirectory/nsapplicationsupportdirectory
     //     on Linux, -> "${XDG_DATA_HOME:-~/.local/share}/com.zulip.flutter/"
     //     All seem reasonable.

lib/notifications/display.dart

@@ -59,7 +59,7 @@ class NotificationChannelManager {
   /// For example, for a resource `@raw/chime3`, where `raw` would be the
   /// resource type and `chime3` would be the resource name it generates the
   /// following URL:
-  ///   `android.resource://com.zulip.flutter/raw/chime3`
+  ///   `android.resource://com.zulipmobile/raw/chime3`
   ///
   /// Based on: https://stackoverflow.com/a/38340580
   static Future<String> _resourceUrlFromName({