lineage-sdk: Add LineageStatusBarItem interfaces

*) Introduce LineageStatusBarItem class that contains interfaces
   and utility methods that can be used by status bar items that
   that are hosted here in the lineage-sdk in order to communicate
   with systemui.

*) Nested interface: Manager
   Implemented by layout view LineageStatusBarItemHolder in systemui.
   Contains methods that are used to request receipt of certain
   systemui events.

*) Nested interface: DarkReceiver
   Can be used to listen for systemui DarkReceiver events.

*) Nested interface: VisibilityReceiver
   Can be used to listen for changes in visibility of a statusbar item.
   Works even if the statusbar item sets it's own visibility to GONE.

*) Util method: findManager(View v) walks up through parent views to
   look for something that implements Manager.

*) Systemui side that provides a layout that implements Manager:
   https://review.lineageos.org/#/c/202265/

Change-Id: Ie237db5bfac9994b87de39372505cd61c39d89ee