[Android] Add StopWatchingAll() for test case ThreadWatcherAndroidTest.ApplicationStatusNotification
ThreadWatcherList::StartWatchingAll creates one static object ThreadWatcherObserver
called g_thread_watcher_observer_. This should be released by ThreadWatcherList::StopWatchingAll.
Like the case ThreadWatcherListTest.Restart at chrome/browser/metrics/thread_watcher_unittest.cc.
If not released, CHECK(!g_thread_watcher_observer_) in constructor of ThreadWatcherObserver would
be failed when running case ThreadWatcherListTest.Restart after this case.
BUG=499723
Review URL: https://codereview.chromium.org/
1182023002
Cr-Commit-Position: refs/heads/master@{#334532}