[Android] Add export check for "game_view_content_description" in strings.xml

[timbotimbo2]

Description

Sometimes a Unity Android export will randomly omit the string "game_view_content_description" from the strings.xml file in the output.

This is the expected output:
android/unityLibrary/src/main/res/values/strings.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <string name="app_name">flutterunitywidgets</string>
  <string name="game_view_content_description">Game view</string>
</resources>

If this string is missing you will get an error like:

E/AndroidRuntime(31563): android.content.res.Resources$NotFoundException: String resource ID #0x0
E/AndroidRuntime(31563): 	at android.content.res.Resources.getText(Resources.java:367)
E/AndroidRuntime(31563): 	at android.content.res.Resources.getString(Resources.java:460)
E/AndroidRuntime(31563): 	at com.unity3d.player.UnityPlayer.GetGlViewContentDescription(Unknown Source:20)
E/AndroidRuntime(31563): 	at com.unity3d.player.UnityPlayer.<init>(Unknown Source:271)
E/AndroidRuntime(31563): 	at com.xraph.plugin.flutter_unity_widget.UnityPlayerUtils$Companion$createPlayer$1.run(UnityPlayerUtils.kt:45)
E/AndroidRuntime(31563): 	at android.os.Handler.handleCallback(Handler.java:883)
E/AndroidRuntime(31563): 	at android.os.Handler.dispatchMessage(Handler.java:100)
E/AndroidRuntime(31563): 	at android.os.Looper.loop(Looper.java:214)
E/AndroidRuntime(31563): 	at android.app.ActivityThread.main(ActivityThread.java:7697)
E/AndroidRuntime(31563): 	at java.lang.reflect.Method.invoke(Native Method)
E/AndroidRuntime(31563): 	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:516)
E/AndroidRuntime(31563): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)

I can find stackoverflow and github issues with this error back to Unity 2018, but I have never managed to reproduce it myself.
Because of that I have no clue what exactly causes this, just that it happens to some people.

This has been popping up again recently here and on Discord, even for Unity 6.

Some old issues and mentions include:

• UnityWidget crashing upon loading #516
• Failed to handle method call : java.lang.NullPointerException #984 (comment)
• [ERROR:flutter/lib/ui/ui_dart_state.cc(198)] Unhandled Exception: PlatformException(error, null, null, java.lang.NullPointerException #651
• Error : android.content.res.Resources$NotFoundException: String resource ID #0x0 Unity-Technologies/uaal-example#48
• https://discussions.unity.com/t/unity-2018-3-6-upgrade-to-unity-2018-4-has-bug/744165/4
• https://stackoverflow.com/questions/56416191/resource-not-found-exception-while-opening-unity-project-in-android-studio

Fix

This change adds the string to strings.xml if it is detected as missing.
It will also log an error to the console if the entire file is somehow missing.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore