[Android] Add a real Activity to simulate Pause/Resume for test

Previously, the case is calling onPause()/onResume of
XWalkRuntimeClient to trigger XWalkView's pause/resume.
After 75b09f33, XWalkView handles the activity status
change within itself. So XWalkRuntimeClient's onPause/onResume
is now empty method.

To test this case, a real Activity "ActivityToCausePause" is
created to trigger the pause/resume event of test target
Activity.