Support split from fullscreen with shortcuts

Recording: https://recall.googleplex.com/projects/f46cfe9c-8076-4efe-bf8a-b1cc4f1f5e1b/sessions/978a3a55-2982-4026-9726-2a73cb694258

TODO: improve transition animation

Bug: 246661484
Test: manual
Change-Id: Id0d1aec0685bc18f4b273f8e466996efc1d9a3d8
Merged-In: Id0d1aec0685bc18f4b273f8e466996efc1d9a3d8
12 files changed