Add a check at the global level for notification bubbles being enabled

None of the checks in NoManService for 'flag bubble' encorporated the
global setting... this CL adds a check for that.

Test changes:
* Moves prefs set up into a helper function so it's clear whether
  bubbles are enabled / disabled at different levels:
  - global
  - app
  - channel
* Adds an additional test to make sure flag isn't applied when disabled
  at app level

Test: atest NotificationManagerServiceTest
Bug: 132699005
Change-Id: I937b6984f4698cfe026d3e34f4808adfb969392d