Make bubble settings a pref with an int rather than a bool

This will allow us to do all/selected/none preferences for
bubbles in settings.

- Feature is on by default
- App is none by default
- Channel is off by default

Test: atest NotificationManagerServiceTest BubbleExtractorTest
Bug: 138116133
Change-Id: Ifad1c22525123354f76959c2d44392a25d56347d