Add multi-display support to shell transitions

This tracks ready state in groups. Currently the groups are
display-contents (to match legacy transitions). The transition
will be considered ready when all groups are ready.

This also adds a couple of inTransition checks in places that
are used by CTS tests.

Bug: 183993924
Test: atest MultiDisplaySystemDecorationTests
Change-Id: Iea81b3c689d948d60aa8e1a2d3bf168723058a2f