Convert HeadsUpManager et al to use an Executor

Note that BaseHeadsUpManager still needs a Handler to register its
ContentObserver to observe the snooze length setting.

Bug: 315192399
Test: atest AlertingNotificationManagerTest
Test: atest BaseHeadsUpManagerTest
Test: atest HeadsUpManagerPhoneTest
Flag: NA
Change-Id: I01c2bf31c88f58dc2aee207845abc61e2611bf97