Add key event handling to Flexiglass

Extracted the key event handling methods from
NotificationShadeWindowView's InteractionEventHandler into a new
class named WindowRootViewKeyEventHandler and moved the key handling
methods in NotificationShadeWindowView up into WindowRootView, the
parent class it shares with SceneWindowRootView.

Test: updated and ran affected tests
Test: manually verified with flag both on and off
Fixes: 399431426
Flag: com.android.systemui.scene_container
Change-Id: I7d2210429c8f1b2b06c42670245d2cc8d88dc3c4