[Android] Allow extension manager to load extension js from res/raw

This is step one of getting rid of assets for xwalk_core_library.
Because assets are not supported well for android library project.

Now XWalkExtensionManager will load extension js files from res/raw
first for internal extensions, then from assets if not found.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1084