android: Don't require aapt to be on the path during builds and tests.

BUG=330631
R=frankf@chromium.org

Review URL: https://codereview.chromium.org/167643002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251452 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251452