Avoid a potential race condition on mDisplays

The race could occur when transitioning in/out of VR flinger mode.
It is now avoided by ensuring that the primary |DisplayDevice| is always
created once |mStateLock| is released, and ensuring that all accesses
to the primary |DisplayDevice| are guarded by |mStateLock|.

Bug: 36194616
Test: Normal behavior is unchanged. Explicitly testing the race
condition required instrumenting code with sleep statements.

Change-Id: I1f8fcf23982c311d47267beca8127b6c29562ce5