Update SessionLayerMap to detect change sets

This patch updates the SessionLayerMap to detect when changes occur
and expose them to create diffs to send as updates. This patch also
adds unit testing for SessionLayerMap.

Bug: 360908317
Test: atest libsurfaceflinger_unittest
Flag: com.android.graphics.surfaceflinger.flags.adpf_native_session_manager
Change-Id: Iee69445987ccf32ab72f96e249b7fa524ca7b4ab