Android: Opt out of back invoked callback

When 9d9b6d8 changed our target SDK version to Android 16, it made
Android 16 stop calling onBackPressed and stop delivering KEYCODE_BACK
events. Dolphin's code isn't ready for that yet.

Android lets us opt out of this new behavior for now, so let's do so.
But the opt-out will presumably stop working once we start targeting
Android 17, so we're going to have to update Dolphin's code within the
next one or two years to support the replacement API.

Source/Android/app/src/main/AndroidManifest.xml

@@ -37,6 +37,7 @@
         android:icon="@mipmap/ic_launcher"
         android:requestLegacyExternalStorage="true"
         android:preserveLegacyExternalStorage="true"
+        android:enableOnBackInvokedCallback="false"
         android:allowBackup="true"
         android:fullBackupContent="@xml/backup_rules"
         android:dataExtractionRules="@xml/backup_rules_api_31"