4-2/ Enhance AbstractStandardAppHelper.exit() to exit PIP at end

Integrate forceStop() in exit() that PIP window could really
exit after each round of test

Bug: 173567664
Test: atest com.android.wm.shell.flicker
Test: atest com.android.server.wm.flicker.Flicker

Change-Id: I8aaa4f590344f8fb1e6e731ed865f21f63892bb2
diff --git a/tests/FlickerTests/AndroidManifest.xml b/tests/FlickerTests/AndroidManifest.xml
index 98e02ce..58f56f2 100644
--- a/tests/FlickerTests/AndroidManifest.xml
+++ b/tests/FlickerTests/AndroidManifest.xml
@@ -27,6 +27,8 @@
     <uses-permission android:name="android.permission.ACCESS_SURFACE_FLINGER" />
     <!-- Enable / Disable tracing !-->
     <uses-permission android:name="android.permission.DUMP" />
+    <!-- Force-stop test apps -->
+    <uses-permission android:name="android.permission.FORCE_STOP_PACKAGES"/>
     <!-- Run layers trace -->
     <uses-permission android:name="android.permission.HARDWARE_TEST"/>
     <!-- Workaround grant runtime permission exception from b/152733071 -->