[Notif] Update HeadsUpManagerImplTest to use `underTest` everywhere.

This way, the individual tests don't need to manually create
HeadsUpManagerImpl each time.

Bug: 378142453
Flag: EXEMPT refactor
Test: atest HeadsUpManagerImplTest
Change-Id: I8b9623c084e19b360d1187e09569420d859d8357
1 file changed