DL manager support for broadcasts + connectivity control

* Additions to DownloadManager to support control of broadcast intents, allowed network types, and roaming control.  This includes new constants and added implementation.
* New DB columns to hold connectivity flags and one to indicate whether the download was initiated through the public API; this is used to change behavior related to broadcast intents and connectivity controls.

Change-Id: I2e8e4ce92cde7889a0f24dc43b86c596c73c0eaa