Adding Flie Playback Function

On "Playback" button clicked, migrate to FileListActivity, in which we
list all media files to user.

When user clicks some file on the list, popup a dialog asking user to
play or delete the file. If the user chooses to play, then migrate to
PlayFileActivity and play it.
Merge branch 'master' of https://github.com/asdfghjkl20203/hs602_player