Avoid a potential race condition on mDisplays

Update: The HWC1 path needed to be updated in light of a change to
SurfaceFlinger.h. The build now works for both paths.

Original Message:

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
Bug: 37249613
Test: Compiled for both HWC1 and HWC2 surface flinger builds.
      Normal behavior is unchanged. Explicitly testing the race
      condition required instrumenting code with sleep statements.

Change-Id: I0b00e857a3b2fd01948a888db2f0715d2a8204c1