Use bg thread for BroadcastDispatcher registering

There is no need for the broadcastdispatcher to register in the main
thread. Moving to the bg thread reduces the chance of our main thread
being busy when many classes subscribe in a short period of time.

Test: manual (logs show BroadcastDispatcher is receiving broadcasts)
Test: atest UserBroadcastDispatcherTest
Fixes: 151684085
Change-Id: Iac6fd9e3b9f7bed3879620b4f262cc68f8ffdcd3