Remove duplicate entries from hiddenapi files

Prevents the hiddenapi files index and metadata files from including
duplicate entries.

Bug: 178361284
Test: m droid
      Compared hiddenapi files before and after change
      Verified that the only differences were due to the removal of duplicates
      Used sort -u to remove duplicates from before file and compared with after file
Change-Id: I0b9e420aec42cdcf4584099a7ffd2ed9c3715d6d