Improve Planner stability

This fixes two crashes in Planner:

1. During device rotation, an Output's layer stack may change, which
invalidates OutputLayer pointers. When updating a LayerState, swap in the
updated OutputLayer pointer as well to keep references from going stale.
2. Retain the set of removedLayers until after Flattener has updated its
internal state in flattenLayers(). Otherwise, removed LayerStates may be
deleted which invalidates dangling pointers in the Flattener.

Bug: 188743022
Test: builds
Test: libcompositionengine_test
Test: rotate Chrome

Change-Id: I889703a2675bc0eddadd344f1a550d93d7f4c8df