[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
12b98a890de27d1ec96eb7f10558010fc7412e58.

BUG=XWALK-2300, XWALK-2301
Merge pull request #2383 from wuhengzhi/backgroud_with_no_instrinsic_size

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