Fixup handling destroy

 Bug: 5002925
 White list certain messages that can be handled after WebViewCore
 was destroyed (specificially pause/resumeTimers, which are per process)
 and reject all other messages. This avoids using the undocumented
 removeCallbacksAndMessages(null) trick, as well as makes sure PAUSE_TIMERS
 and RESUME_TIMERS are processed in the correct order the correct number
 of times.

Change-Id: I8c51172e015f923ffeb0816a8ee2ab63219d0f96