libgui: add helper to find CpuConsumer::AcquiredBuffer

Add CpuConsumer::findAcquiredBufferLocked.  Rename mBufferPointer to
mLockedBufferId.

This also fixes a nullptr-dereference when unlockBuffer is called
with a LockedBuffer whose data is nullptr.

Test: libgui_test
Change-Id: I36c665099f145731197a6ff8ab940c5d7a326df8
3 files changed