[Android] Include "-DOS=android" in xwalk_android_gyp by default

In M36, android_gyp is still available in upstream, but it is
going to be removed in recent future.
We can still use it for crosswalk 7 with appending "-DOS=android"
by default in xwalk_android_gyp.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1803