[Notifs] Rename BaseHeadsUpManager to HeadsUpManagerImpl.

Now that there's only one implementation of HeadsUpManager, we should
name its implementation HeadsUpManagerImpl.

Bug: 378142453
Flag: EXEMPT refactor
Test: compiles
Test: atest HeadsUpManagerPhoneTest HeadsUpManagerImplTest
Change-Id: I5166eb73816118a1e6d9124afacd8b5f29f88bc2
9 files changed