android: Make buildbot able to use a custom manifest.

In order to support rolling to a new version of AOSP to run gyp tests
(to pick up android build system changes), introduce logic in the
buildbot script to compare the current manifest being used with the one
in the gyp tree and resync/compile android if it changes. Check in an
initial version of this manifest which is pinned to a recent revision of
AOSP's master branch.

BUG=gyp:324
R=jbudorick@chromium.org, torne@chromium.org

Review URL: https://codereview.chromium.org/493743002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1968 78cadc50-ecff-11dd-a971-7dbc132099af