Add support for sending files from app UI

 * Turned FileManager into an activity so we can use startActivityForResult
   This can be used elsewhere in the app also. This works for the time
   being, I think setting it back as a fragment and implementing a callback
   would be best. Current method may introduce issues with stopping,
   pausing and resuming the main activity.
 * Works, needs a little cleanup.
 * Started work for a dual-use file manager activity
   ( used for getting files, and setting the download directory. )

Change-Id: Icd141db2f4212a8700f78c1f4342268699356b8b