Android: use relative paths for action/rule/copy names.

qualified_target is an absolute path; create a relative version for use
when generating names of actions/rules/copies. This avoids including
the absolute path of the source tree in the makefiles, making
regeneration cleaner.
Review URL: https://codereview.chromium.org/11967045

git-svn-id: http://gyp.googlecode.com/svn/trunk@1563 78cadc50-ecff-11dd-a971-7dbc132099af