[Android] Add "xwalk" prefix to manifest extensions

According to http://w3c.github.io/manifest/, vendor
extensions should have vendor prefix.
For Crosswalk on Android, launch screen is an extension
of manifest.
This commit add "xwalk" prefix to all related paths.
The legacy paths are still supported, but will print a
warning message to logcat.

Related to XWALK-1918
indentation not needed here.