Fix issue #6381224: Initial emulator boot fails and shows a blank black screen.

Make sure that all cases where we remove an activity from the history
stack, we call resumeTopActivityLocked() to cause the home activity
to be launched if the stack is now empty.

Also fixed a problem where some timeouts would not be removed when destroying
an activity, and a race condition in boot that would cause the
PhoneWindowManager to initially start out with the home key not working.

Bug: 6381224
Change-Id: If046bb01aed624b0d9ee3bbaaba68ed6b98fd1d0