not allow users to modify emergency alert notification channels

1. Due to the nature of emergency alert and regulation requirements
, users should not block the notifications from cellbroadcast app.
2. Users can be confused about this setting. User may expect it will
not play any siren sound if it's set as Slient. However this settings
only control the notification behavior and the actual siren alarm sound
is played & controlled inside cellbroadcast app.

Bug: 177296238
Test: Manual
Change-Id: I379b5a2e2a8d92861385e0f13e0db6f39fdec7ca