add "junk path" -k option to aapt

Adds a "junk path" option to aapt so that you can do:

   aapt add -k archive.zip path/to/some.file

and have the file stored in the zip under the name "some.file"
(without the path).  Needed so that we can use 'aapt add' in place of
'zip -j' when building jar files, which will lead to smaller
incremental OTAs.