DO NOT MERGE Prevent apps from creating blocked channel groups

setBlocked is a hidden API, so apps should not be calling
the method, but fix up the data in case they do

Test: PreferencesHelperTest; manual with ApiDemos FGS
Bug: 209966086
Change-Id: Icc709a6b0d0a8c5f2d9243959992f1b6764354db
Merged-In: I8a27853c7ed05d9dfd38a3142fbbe185946c3992