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

Two main changes in this commit:
1 - Resolves the issue with really long sync times due to the scan for media changes
2 - Exposes a "check media" option in the menu

The gist of this commit:
The media scan is removed from the sync step (except on the first sync) but can be triggered manually with a media check which is now an option in the menu. Changes to media made through AnkiDroid (e.g., adding image through multimedia editor) are immediately recorded in the media database, so a scan for changes is not necessary unless the media collection is modified externally. In that case, the user will need to do a media check.