Require DownloadRequest#Builder to have mandatory Source URI

This change modifies the API to require that the
DownloadRequest#Builder includes the Source URI as a mandatory,
non-null parameter.

Test: Manual, Telephony Test MBMS app
Bug: 67640102
Merged-In: I25b39008317e333d422cd32a0c6348c010ca280f
Change-Id: I7d44e977314a57fdf063aa233bfb978b48ebf1db