[Notifs] Create notification.headsup package; move HUN files there.

Moves BaseHeadsUpManager and a lot of other heads up classes to a new
statusbar/notification/headsup package.

Bug: 378142453
Flag: EXEMPT refactor
Test: compiles
Test: atest HeadsUpManagerPhoneTest BaseHeadsUpManagerTest
Change-Id: I40231ab2c987bd7bd3b7da7fa87ba48a4431281a
93 files changed