Allow updates to importance of default apps

As long as it's not IMPORTANCE_NONE

Test: manual
    1. Messages app > Settings > Notifications
    2. Change behavior from Prioritized to Gentle
    3. Back
    4. Click on Notifications again
    5. Observe: setting was changed to Gentle
Test: atest PreferencesHelperTest
Fixes: 131893709
Change-Id: I1710435e9dfa741b14a8de3da07bff1d17e884b8