Sync call log entries to secondary user on background thread

Add code (similar to CP2) to offload tasks onto a background thread. This
is a bit overkill for now but will be useful when we add stuff like call
log syncing.

Also add some new methods to MockUserManager needed to pass tests.

Bug: 18056687
Change-Id: If51f64895d16029ecca1d1c46573324891e1d4d0
2 files changed