Fix missing check on buffer import.

Fix a missing check on the success of importing buffers into the
ConsumerQueue. There is a race condition where the producer can
invalidate its buffers, for example by resizing, before the consumer
has a chance to import the previous buffers. The missing check
causes a crash in SurfaceFlinger and VrCore, which are both
consumers of application VR surface buffers.

Also fix a missing lock around the consumer queues in VR surfaces
found during the analysis of this bug.

Bug: 64042620
Test: Ran test.apk before and after the fix. Observe stable operation
      after applying the fix.

Change-Id: I416df3ca47978404dcdb53599ddeec9b4bd6fb1a