Allow apps to downgrade importance in createNotificationChannel

Only takes effect if the user has not changed any fields.
Some tangential cleanup of the test file:
 - Rename member variable "uid" to "mUid"
 - Use correct PKG instead of "test_pkg"
 - Some character limit fixes

Test: runtest systemui-notification
Fixes: 63755460
Change-Id: I3c2df0c03ec333f320ac7a0fe90792f1b0ef02ce