[SB][Notif] HeadsUpApperanceControllerTest improvements

 - No Hungarian notation
 - Mocks created at variable declaration
 - Variables created in `setUp` are now `lateinit` so they aren't
   nullable
 - Use `underTest`

Bug: 378142453
Flag: EXEMPT refactor
Test: atest HeadsUpApperanceControllerTest
Change-Id: Ibd8d1ce0104c0ad450f1a655e1ac3165ec8ee481