Simplify Startup Initializers

Since the project current modularization does not yet allow to split bindings and the rest of the code, moving the `Initializer`s registration to the `:app` module makes it easier to use.

- It avoids duplicated registrations (it was both present in `:app` and `:core:network`)
- It avoids removing registrations on other modules that would break during instrumented tests

`EnsureInitializerMetadata` had to be suppressed because the Lint rule does not actually check where the initializer are registered, and always expects to see it on the same module, which is quite a simplistic assumption.
https://googlesamples.github.io/android-custom-lint-rules/checks/EnsureInitializerMetadata.md.html

I also remove the hardcoded `androidx.startup` string with the proper string reference `@string/androidx_startup`.

Author: SimonMarquis

app/src/main/AndroidManifest.xml

@@ -58,13 +58,13 @@
             tools:node="merge">
             <meta-data
                 android:name="com.android.developers.androidify.startup.FirebaseAppInitializer"
-                android:value="androidx.startup" />
+                android:value="@string/androidx_startup" />
             <meta-data
                 android:name="com.android.developers.androidify.startup.FirebaseAppCheckInitializer"
-                android:value="androidx.startup" />
+                android:value="@string/androidx_startup" />
             <meta-data
                 android:name="com.android.developers.androidify.startup.FirebaseRemoteConfigInitializer"
-                android:value="androidx.startup" />
+                android:value="@string/androidx_startup" />
         </provider>
         <activity
             android:name=".MainActivity"

core/network/src/main/AndroidManifest.xml

@@ -15,6 +15,7 @@
  */
 package com.android.developers.androidify.startup
 
+import android.annotation.SuppressLint
 import android.content.Context
 import androidx.startup.Initializer
 import com.google.firebase.appcheck.FirebaseAppCheck
@@ -25,6 +26,7 @@ import com.google.firebase.ktx.Firebase
 /**
  * Initialize [FirebaseAppCheck] using the App Startup Library.
  */
+@SuppressLint("EnsureInitializerMetadata") // Registered in :app module
 class FirebaseAppCheckInitializer : Initializer<FirebaseAppCheck> {
     override fun create(context: Context): FirebaseAppCheck {
         return Firebase.appCheck.apply {

core/network/src/main/java/com/android/developers/androidify/startup/FirebaseAppCheckInitializer.kt

@@ -15,6 +15,7 @@
  */
 package com.android.developers.androidify.startup
 
+import android.annotation.SuppressLint
 import android.content.Context
 import androidx.startup.Initializer
 import com.google.firebase.Firebase
@@ -24,6 +25,7 @@ import com.google.firebase.initialize
 /**
  * Initialize [FirebaseApp] using the App Startup Library.
  */
+@SuppressLint("EnsureInitializerMetadata") // Registered in :app module
 class FirebaseAppInitializer : Initializer<FirebaseApp> {
     override fun create(context: Context): FirebaseApp {
         return Firebase.initialize(context)!!

core/network/src/main/java/com/android/developers/androidify/startup/FirebaseAppInitializer.kt

@@ -15,6 +15,7 @@
  */
 package com.android.developers.androidify.startup
 
+import android.annotation.SuppressLint
 import android.content.Context
 import android.util.Log
 import androidx.startup.Initializer
@@ -27,6 +28,7 @@ import com.google.firebase.remoteconfig.remoteConfigSettings
 /**
  * Initialize [FirebaseRemoteConfig] using the App Startup Library.
  */
+@SuppressLint("EnsureInitializerMetadata") // Registered in :app module
 class FirebaseRemoteConfigInitializer : Initializer<FirebaseRemoteConfig> {
     override fun create(context: Context): FirebaseRemoteConfig {
         return Firebase.remoteConfig.apply {