Add SMS Middleware layer.

Change-Id: If5621d0c5126d6917ab00143af645692f3c4a384
SMSDispatcher: Fix determining the defaultSmsApp for proxied messages

We can't rely on the calling UID, since for re-broadcasted texts that'll
always be the telephony itself; so if called by the framework, use
the sentIntent creator to determine which app originated the message

Change-Id: I60ac02f6ae09f4260f5494659a174352f353aa2a