[flexiglass] Transitions for communal

Adds transition definitions between communal and shade and between
communal and bouncer.

Bug: 339449476
Bug: 323068793
Test: manually verified that the transitions are animated, even if not
perfectly
Flag: com.android.systemui.scene_container

Change-Id: I53ce0ed303aa2e10cd3b330b37dd1f627c00f592