SurfaceFlinger: Avoid destroying Layer on Binder thread

BufferQueueLayer::onFrameAvailable passes 'this' as an sp<Layer>
to SurfaceInterceptor. This constructs a temporary sp<Layer>. We are
on a binder thread and not holding any locks, so at this point the main
thread could drop it's last references. Then when we destroy our
temporary sp<Layer> it is the last reference and we end up
invoking ~Layer from the Binder thread, an invalid operation
which in this case leads to dead-lock (as we attempt to reacquire
the already acquired BufferQueue mutex from the BufferQueueLayer d'tor)

Bug: 149473038
Test: Existing tests pass
Change-Id: I77a20bedf2db3b974ac03d804f70993514478fb2