Fix bug where FSI decision was not logged unless FSI was launching.

* Change HeadsUpCoordinator's `onEntryAdded` logic so that `logFullScreenIntentDecision` is called unconditionally after `getFullScreenIntentDecision`.
* Change HeadsUpCoordinator's `onEntryUpdated` logic so each reconsideration can keep happening ONLY if the decision keeps being `NO_FSI_SUPPRESSED_ONLY_BY_DND`.
  * If the FSI should launch, in addition to launching it still calls `logFullScreenIntentDecision` and now also revokes the candidacy for reconsideration.
  * If the FSI should NOT launch (and the decision changed), it now calls `logFullScreenIntentDecision` and also revokes the candidacy for reconsideration.
* Fixes a bug & fragile code in `logFullScreenIntentDecision` where not all decision values were enumerated for logging.  The "human readable" string is gone, and replaced with a default case which logs the enum name.
* Adds some nullability annotations to the java interface.
  * Remove an unnecessary null check on the decision.
  * Convert an enum `.equals()` to `==`.

Bug: 265977861
Test: dumpsysui NotifInterruptLog
Test: atest HeadsUpCoordinatorTest NotificationInterruptStateProviderImplTest
Change-Id: Ie6d379008c9cd99cfcfe0f9da43e602a436a0d39