PlaylistEditFragment: Fix playlist file track removals.

This commit fixes a regression from ef2484a where playlist files were given a
position as a song id, since they don't have a natural unique id by design.
This didn't make sense from a JMPDComm standpoint, so it got removed. Then I
found the regression which playlist track removals required a unique songid
so, this patch implements playlist file track position as a UID in MPDroid.