Remove BroadcastReceiver ref on unregister

There were unremoved references in a map in UserBroadcastDispatcher that
were not being removed on unregister, leaking objects.

Test: atest UserBroadcastDispatcherTest
Test: hprof
Fixes: 144937054
Change-Id: Icc33a28711eadcd0d566c468f34e76a3913700f3