Hold mMutex lock and check abandoned when accessing mLayer

The Layer lifecycle is tied to BufferLayerConsumer so when the Layer is
removed, the BufferLayerConsumer is abandoned. However, abandoned
doesn't necessarily mean it was cleaned up yet. Therefore, we need to
check whether the BufferLayerConsumer was abandoned before accessing the
raw pointer mLayer. If the BufferLayerConsumer was not abandoned, then
mLayer will be valid.

Test: Builds and runs. Hard to reproduce race condition
Fixes: 157535966
Fixes: 155679049
Change-Id: I8b309c9e1fe57746bceabab1deda56248087b189