Address communal UI glitches.

This changelist addresses two sources of UI glitches:
- The CommunalHostViewController is now initialized
  once, preventing multiple, potentially stale, signals
  to the communal code.
- Bouncer presence has been removed from the invalid
  states as the communal view can be shown as the same
  time as the bouncer during transition.

Bug: 195601027
Test: atest CommunalHostViewControllerTest#testNoShowInvocationOnBouncer
Change-Id: I64893d9e57cba8dbbdb6085e7a6e2b677455f273