surfaceflinger: make vsync injection more robust

There are more issues than I expected :)

 - no lock to synchronize enable/disable and injection
 - Every time injection is diabled and enabled, a new EventThread is
   created
 - mCallback might be nullptr
 - ENABLE_VSYNC_INJECTIONS/INJECT_VSYNC should require special
   permission
 - MessageQueue::setEventThread must be called from the main thread
 - MessageQueue::setEventThread does not handle EventThread switch
   well

Bug: 65483324
Test: manual
Merged-In: I7d7b98d1f57afc64af0f2065a9bc7c8ad004ca9f
Change-Id: I7d7b98d1f57afc64af0f2065a9bc7c8ad004ca9f