Turn off glanceable hub touch handling on dream if disabled

- Added an `isEnabled` field in DreamTouchHandler interface which
  defaults to true.
- If communal is not available, set enabled state to false in
  CommunalTouchHander, so that no touch session or region
  will be initiated trying to handle touch for the glanceable hub.

Bug: b/323366236
Test: edge swipe on dream with GH enabled/disabled
Test: atest CommunalTouchHandlerTest
Flag: ACONFIG com.android.systemui.communal_hub STAGING
Change-Id: Ifa5941c27aa7dc55e25f0e3277b1e5cedde38838