[Android] Add support of getting mime type for app url scheme.

This patch is to add support of getting mime type for app url scheme.
As crosswalk supports app scheme and the local path in "iframe" was
loaded with "app://", so add conditional to get the related mime type.
The app scheme was removed in commit
12b98a8.

BUG=XWALK-2300, XWALK-2301