Ignore onPostListReady call if the activity has been destroyed

ResolverActivity#onPostListReady is called asynchronously on the main
thread and it is possible for it to be executed after activity’s
onDestroy call.
A check for activity’s status is added to the method.

Test: atest FrameworksCoreTests:ChooserActivityTest
Fix: 254787909
Change-Id: I96d98a03d86881845868a67e33ea8ee7c9dc6631