Ensure onHandleDestroyed drops reference before releasing lock.

onHandleDestroyed copies the layer pointer and references it in
mLayersPendingRemoval, so that the final reference will be dropped
from the main thread. However the current implementation is flawed.
It is possible for the main thread to process mLayersPendingRemoval
after we release the lock, but BEFORE we finish executing the
~LayerCleaner destructor. This would mean the last reference would
again be LayerCleaner::mLayer and we would incorrectly run the d'tor
on the main thread. Scheduling priority boosts to the main thread
make this more likely than it may seem.

Test: Boots. Existing tests pass.
Change-Id: Ife247b530b67907b28b4f29f6766c2056fe49bfa