Remove mutex from file seselector

mutex would, in some rare cases fail to lock and cause a freeze
of the GUI.

Also restored some code that was removed before and should not
have been removed to fix a bug with folder selection where you
select the same file or folder twice (e.g. select a backup on the
restore page, press back, then select that backup again.)
Merge "Remove mutex from file seselector" into twrp2.4