BufferQueue: release mutex while allocating. DO NOT MERGE

BufferQueueProducer::allocateBuffers used to keep the BufferQueueCore
mutex while doing the buffer allocation, which would cause the consumer
(which also needs the mutex) to block if the allocation takes a long
time.
Instead, release the mutex while doing the allocation, and grab it again
before filling the slots. Keep a bool state and a condvar to prevent
other producers from trying to allocate the slots while the mutex is
released.

Bug: 11792166

Change-Id: I4ab1319995ef892be2beba892f1fdbf50ce0416d
(cherry picked from commit ea960444)