Notify apps when channels/groups are blocked/unblocked

So that apps can disable/enable receivers/jobs/etc. that
would only need to run to post notifications to the blocked
channel(s).

Additionally let apps retrieve an individual group so they
can inspect blocked state.

Change-Id: I733b70c62cd0482d0cf9692ea9b00cf313ad7b81
Fixes: 36530302
Test: runtest systemui-notification, cts