[Notif] Fix most of the IDE warnings in HeadsUpManagerImpl.

Fixed most of the warnings from the IDE on HeadsUpManagerImpl so that
future changes will start from a cleaner place. Most changes are
explicit null checks, plus some simplifications.

Bug: 378142453
Flag: EXEMPT refactor
Test: SysUI builds, atest HeadsUpManagerImplTest
Change-Id: I426d8365e4292df89f95c39ca2ac7b4dfe5728b9