[flexiglass] Bouncer action button ("Emergency Call"/"Return to Call").

Bug: 298026988
Test: Manually tested that the button shows and behaves as expected
 during a phone call ("Return to call"), during idle bouncer state
 ("Emergency"), and when the device has an insecure auth method
 (no button should be shown).
Test: Added new unit tests.
Test: Added new integration tests.
Test: Existing unit tests still pass.
Test: Existing integration tests still pass.
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Change-Id: I6fd0dcaa2ddfdc7fc35cb03b7687b687b1672e2c