Remove legacy layer hierarchy

Legacy layer objects are now all stored in the mLegacyLayer map. They
are created on the binder thread and destroyed on the main thread.
Cloned layers do not have a legacy layer.

This change also keeps the create layer alive until main thread wakes up
to make the lifetime a bit simpler to track.

Flag: EXEMPT flag removal
Bug: 330785038
Test: presubmit
Change-Id: I4132e7a7c373c823ad914209955f7b0f55af008d