[flexiglass] suspend fun activate now returns Nothing

This will help catch issues at compile/IDE time where developers can
write unreachable code if they put it after a call to a super.activate()
which suspends forever.

Bug: 354270224
Test: smoke test - ran Flexiglass, moved through all scenes
Flag: com.android.systemui.scene_container
Change-Id: I2b2d0bc8e141d2e8cbd218098339975b6fecf48e
27 files changed