[SB][Notif] Convert HeadsUpAppearanceControllerTest to Kotlin.

Does some minor cleanup like using `org.mockito.kotlin` for mocks and
`assertThat` for tests. Will do additional cleanup (no hungarian
notation, using Kosmos, etc.) in future CL.

Bug: 364653005
Flag: TEST_ONLY
Test: atest HeadsUpAppearanceControllerTest
Change-Id: If81dba05c7129b28477d0026bbb232b0b5e2c2f3