Don't allow setting channels to bypass DND if the app cannot show notifications

The fix makes the flow more consistent with preexisting behavior:
* Apps appear in the "Apps that can interrupt" section only if some channels are set to bypass DND *and* the app is able to show notifications.
* Channels cannot be set to bypass if those individual channels are blocked.

Fixes: 265064188
Test: atest AppChannelsBypassingDndPreferenceControllerTest
Change-Id: If10f086fd60f0f70f53adb8f5cd67f90936bc35f