[CB-2435] Added a class for general file methods.

The File plugin class has been renamed to FilePlugin.  FileUtils is now a general file utility class (as it should be).
[CB-2435] Fixed getMimeType for content:// URIs.
[CB-2435] Made getRealPath take a URI string.
[CB-2435] Updated the FilePlugin LOG_TAG.
[CB-2435] Added basic android_asset handling.

There is no such thing as a "real path" for a file:///android_asset URI.  However, it is possible to get an input stream to one.
[CB-2435] Made some minor changes.

- removed unused FileReader/FileWriter instance variables
- added logging when getRealPath fails
- fixed indentation issues
- removed a try/catch in favor of throwing
- removed a null check in favor of throwing
- moved getEntry back to FilePlugin
[CB-2435] Renamed file-related classes.

The original FileUtils has been reverted back to using that name (instead of being changed to FilePlugin).  The new helper class is now called FileHelper (instead of FileUtils).