releasetools: Use java_path in sign_target_files_apks.py.

Prior to this CL, it was calling the hard-coded "java" although it was
accepting a "--java_path" option.

Also switch OPTIONS.java_args from string to list. Otherwise it won't
work when providing multiple args.

Bug: 32737832
Test: Specify "--java_path=" and "--java_args" when invoking
      sign_target_files_apks.py with "-v". Check the commands being
      called.
Change-Id: Id7ef98e778646d532027434de7fba9b7a104dbd0