Fix custom font loading on Android for resource mapped font files

1: Use the CCFileUtils::fullPathForFilename function to lookup the resolved path for the font in the Android specific function, CCImage::getBitmapFromJava.

2: Remove a warning previously added in pull request 2422 (https://github.com/cocos2d/cocos2d-x/pull/2422) in CCFileUtils::fullPathForFilename in order to support change (1) above. If we pass just a system font name to CCFileUtils::fullPathForFilename such as 'Arial' (instead of a path) then it should just return 'Arial' and not generate a warning. We can't assume that when a path (or handle) given to CCFileUtils::fullPathForFilename isn't found within Cocos2dx that it doesn't exist or isn't valid. Depending on the platform there might be files or folders or handles that could be accessible to the app which might exist outside of the knowledge of Cocos2dx. The correct place to warn about missing resource files would be at the point where the resource loading itself fails; e.g if a texture or sound fails to load for example.