API: Expose whether bubbles are enabled or not

There's no way for devs to check whether the user has
disabled or enabled bubbles at the feature level. This
adds an API for devs to make this query.

Test: atest NotificationManagerTest NotificationManagerServiceTest
Bug: 171162184
Change-Id: I3b201e8267f36b45473987db773c08663b8d8126
5 files changed