Make GameSessionTrampolineActivity invisible

This change adds a new theme to prevent
GameSessionTrampolineActivity from displaying
any UI, which in turn delegates the
decision of whether or not to show things
like the system bars to the delegate
activity. This change also disables
transition animations for the trampoline
activity, to prevent them from causing
flickering.

Bug: 229757156
Test: manual testing using GMS dashboard
Change-Id: I82e0ede2e9f0b334fd01e958eab0736042119e84