Don't scan media on every sync and expose media checking to UI

The media checking functionality has been completed and exposed to the user in the UI. 

With this commit, AnkiDroid will no longer scan (or even attempt to scan) for media changes on every sync. Changes made to media through AnkiDroid are recorded in the media database directly. If the media directory is modified externally (e.g., with a file manager),  the user must use the media checking feature to update the media database by scanning all files again.

Other changes:
 - Adding media files through imported decks also mark file additions in the media db
 - The multimedia note editor now uses the media manager to add files instead of copying files directly (which marks additions in the media db)
 - Fixed issue where non-temporary media wasn't copied over in multimedia note

Includes commits:
display more feedback when syncing media (https://github.com/dae/anki/commit/cf801e4fb4c9792347192c8c78c09bcac652ba47)
we need to count up for downloads (https://github.com/dae/anki/commit/3ee193731007ca8df28cbe9534218a70d97599fa)
Merge pull request #484 from hssm/media-changes-check2

Don't scan media on every sync and expose media checking to UI