Fix concurrent access bug in VrFlinger.

VR surface queues have erroneous concurrent access from two threads
because of misplaced lock guards. Move the locking to cover the
queue accesses.

Bug: 62886596
Bug: 63149525
Test: Extensive logging shows that buffers are accessed in the
 correct order and that the consistency of the queues is now
 maintained.

Change-Id: I649e3d75a2230cf3aff50886df9958917d3aa4bf