[SB][Notif] Refactor HeadsUpAppearanceController to use PinnedStatus.

No functional changes in this CL. This CL just updates
HeadsUpAppearanceController's internals to use PinnedStatus instead of a
boolean to keep track of the current heads up status. This will be used
in a future CL, where a notification could be pinned in different ways.

Bug: 364653005
Flag: EXEMPT refactor
Test: trigger HUN -> verify status bar behavior
Test: atest HeadsUpAppearanceControllerTest
Change-Id: I9a5db6a2b95ef8779a2bc6ed3302412ed2591d29