[Notif] Merge HeadsUpManagerPhone into BaseHeadsUpManager.

Arc used to have its own implementation of BaseHeadsUpManager, which is
why HeadsUpManagerPhone was originally created. Since the Arc
implementation no longer exists, we can combine BaseHeadsUpManager and
HeadsUpManagerPhone into a single class to reduce cognitive load.

Also adds explicit @Override to methods that are implementing the
HeadsUpManager interface, and moves Javadoc from HeadsUpManagerPhone to
the HeadsUpManagerinterface. Otherwise, tries to leave the existing code
unchanged.

Bug: 378142453
Flag: EXEMPT refactor
Test: SysUI + all variants compile
Test: Key notification CUJs work:
 - Receive notif while unlocked -> notif HUNs
 - Swipe up on HUN -> notif snoozed
 - Swipe left/right on HUN -> notif dismissed
 - Tap on HUN -> launches activity
 - Receive notif while locked with bypass enabled -> notif HUNs
 - Receive notif while locked with bypass disabled -> notif becomes part
   of stack
 - Enable notification cooldown -> notifs cool down

Test: atest BaseHeadsUpManagerTest HeadsUpManagerPhoneTest
Change-Id: Id1722d845c910f95e9ec37c39f67195ddbf7226f