[DO NOT MERGE] Fix BroadcastDispatcher registering with CURRENT

When a BroadcastReceiver is registered with UserHandle.CURRENT, the
current user should be obtained. This has to be cached in order to not
call ActivityManager every time.

As it is done in the initializer the initial value caching will happen
before any other BroadcastReceiver is actually registered. The caching
and updating is all done in the background thread.

Also, remove unnecessary dependency.

Test: manual
Test: atest BroadcastDispatcher
Fixes: 158298581
Change-Id: Idfd2dc65f9079c57e971250aa7b0aef222da5af5