Make SmsApplication handle multiple users

Assume that there's one default app per user. Apps should be able
to register as the default app for even secondary users.
Read and write to the correct settings db.

Deliver DELIVER broadcast to primary user.
Deliver RECEIVED broadcast to all permitted and currently
running users.

Bug: 16681533
Change-Id: I08cf5318f267a8c55ac427ac2c3573b4319b5193