[flexiglass] Scene class hierarchy changes

1. Removes the default implementation of activate from the Scene
   interface. awaitCancellation nodes are actually expensive. Removing the default
implementation from the interface forces each scene class to implement a
better node.
2. Changes scenes to extend ExclusiveActivatable instead of Activatable
   directly so they can benefit from the reentrant protection
3. This also led me to discover an oversight where the QS shade scene
wasn't activating its actions view-model, so win-win.

Bug: 354270224
Test: manually verified that all scenes can be navigated to and between
Flag: com.android.systemui.scene_container
Change-Id: I9d88012226ac2ed8b2fb95179e38cdc48623adde