[flexiglass] The shade scene returns to Communal if it came from there

Bug: 323068793
Test: manually verified that swiping up on the shade returns to the
right scene. If it came from Lockscreen, it goes back to lockscreen, if
it came from Gone, it goes back there, and most importantly, if it comes
from Communal - you guessed it :)
Test: unit test added
Flag: com.android.systemui.scene_container

Change-Id: I02b05703de5a4a2695e687c1db4d52cff65ddcdb