Ensure that Android embedding initialization always uses an application context (#16204)

Some parts of the embedding (e.g. VsyncWaiter) may hold global references to
system services obtained through the context used during initialization.
These must not be associated with an activity or other non-application context.

Fixes https://github.com/flutter/flutter/issues/49612

shell/platform/android/io/flutter/embedding/engine/FlutterEngine.java

@@ -215,7 +215,7 @@ public class FlutterEngine {
       boolean automaticallyRegisterPlugins
   ) {
     this.flutterJNI = flutterJNI;
-    flutterLoader.startInitialization(context);
+    flutterLoader.startInitialization(context.getApplicationContext());
     flutterLoader.ensureInitializationComplete(context, dartVmArgs);
 
     flutterJNI.addEngineLifecycleListener(engineLifecycleListener);

shell/platform/android/io/flutter/embedding/engine/loader/FlutterLoader.java

@@ -113,6 +113,9 @@ public class FlutterLoader {
           throw new IllegalStateException("startInitialization must be called on the main thread");
         }
 
+        // Ensure that the context is actually the application context.
+        applicationContext = applicationContext.getApplicationContext();
+
         this.settings = settings;
 
         long initStartTimestampMillis = SystemClock.uptimeMillis();

shell/platform/android/test/io/flutter/embedding/engine/FlutterEngineTest.java

 package test.io.flutter.embedding.engine;
 
+import android.content.Context;
 import io.flutter.plugin.platform.PlatformViewsController;
 import io.flutter.plugins.GeneratedPluginRegistrant;
 import java.util.List;
@@ -21,6 +22,7 @@ import io.flutter.embedding.engine.loader.FlutterLoader;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.atLeast;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
@@ -102,4 +104,19 @@ public class FlutterEngineTest {
     // AKA hot restart.
     verify(platformViewsController, times(1)).onPreEngineRestart();
   }
+
+  @Test
+  public void itUsesApplicationContext() {
+    Context context = mock(Context.class);
+
+    new FlutterEngine(
+        context,
+        mock(FlutterLoader.class),
+        flutterJNI,
+        /*dartVmArgs=*/new String[] {},
+        /*automaticallyRegisterPlugins=*/false
+    );
+
+    verify(context, atLeast(1)).getApplicationContext();
+  }
 }