[flexiglass] Makes window focusable.

Added minimal logic to use NotificationShadeWindowController to keep the
focus flags of the window view up-to-date as scenes changes.

Fix: 316616773
Test: added unit test
Test: manually verified that we get to the RESUMED state in
SceneContainer's @Composable function, when we didn't before **for the
shade showing up on top of the unlocked device** (it was already doing
that for the locked device)
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I757cf558f32c06d6b783ea6ca4c07663d7e19c78