Fix calculation of areChannelsBypassingDnd

The value in PreferencesHelper might be wrong on boot (always false, because it was calculated before loading channel info) or after switching users (retained the value from the previous user). This situation would persist until a channel's priority was manually updated.

Also renamed the field in PreferencesHelper to make it clearer that the value corresponds to the current user.

Test: atest + manual
Fixes: 280848017
Change-Id: I1d20339f04f3b27404a0aaeffb563cc5023120d8