[DO NOT MERGE] GraphicBufferAllocator: stall alloc for async frees

This change makes GraphicBufferAllocator::alloc wait for pending async frees to
complete before attempting to allocate a gralloc buffer if there are more than
8 pending async frees.

Bug: 7696861
Change-Id: I1fae86e13edefcaa153b8ce9fd057f335716059e