SF: log fatal if layer destroyed off main thread

We're seeing occasional nullptr errors when LayerFE accesses its
mSnapshot member during SurfaceFlinger::composite calls. One way this
may happen is if a Layer is destroyed during SurfaceFlinger::composite,
causing LayerFE to receive a null snapshot. Adding these logs should
help to identify the root cause and prevent LayerFE::mSnapshot nullptr
errors.

Bug: 261627672
Test: presubmits

Change-Id: I83b3fa304c65ae2dbe936f0f2bdc08045ed96d1f