Expose NotificationManager#getBubblePreference

Previously we were only able to expose whether bubbles
were "all" allowed rather than the selected state. Since
we can change APIs again lets expose the specific prefs.

Bug: 170267239
Test: atest NotificationManagerTest in CTS CL
Change-Id: Ie83934fe1a6b1639fad54967b67e86f6eeeff7cf