Provide onDeactivated method

During refactor I've noticed that many places use try/finally in
onActivated to do cleanup work. This was required before as we had to
return Nothing but this construct is unintuitive.

A onDeactivated method is easy to read and write even without any
knowledge about the Activatable API.

Bug: b/420591935
Test: Refactor only
Flag: com.android.systemui.scene_container
Change-Id: I81feff6a4e992b5f26714dd67f741e1c8842d033