Update extract-files for updated apk entries

* a7ebbbb218ddb9a0102dc46eb1155e0c318f46fe
  'hammerhead: Update makefile generation script'
  added a hyphen in front of the apk entries but
  extract-files.sh was not updated to deal with that,
  breaking the script.
* Make it remove the leading hyphen as well.

Change-Id: I3be3096ca69aeca8303095e88d2a4f2a708fe782