Clean up duplicate shadow lengths in layer snapshot

The shadow length is currently tracked in both the LayerFE
CompositionState and the shadow settings, which are used by the
RenderEngine. We can consolidate these fields and track shadow
settings in the LayerFE CompositionState. This makes sense because
we want the LayerFE CompositionState to contain all relevant
information that it can pass to the RenderEngine without calling
back into the frontend.

Bug: 302551905
Test: presubmit

Change-Id: I583c43993cf73784c6fec9ec2d40f2c76d21adeb