Finish Attachment support for EAS accounts.

* Change service API to allow caller to supply complete target path/file
* Also allow caller to supply the final content_uri
* In MessageView, use full integration with EAS service API and
  attachments content provider to enable:
  * Save: Only works on SD card
  * View: Works w/o SD card using content provider & intents
  * Thumbnail previews
4 files changed