soong_zip: add support for -j to junk paths

When -j is specified ignore the path to the source file and just
zip it with its filename.  -j overrides -C, and -C overrides -j,
so -j -f path1 -C dir -f path2 will junk the path for path1, but
treat path2 as relative to dir.

Remove the filepath.Clean for the FileArgs, it would convert ""
to "." sometimes, and everything gets cleaned in zip already for
the non-command-line use cases.

Test: m checkbuild
Change-Id: I7d90572c622ee0c6f35967ff31d067b5312c72eb