Add default implementation of getLength to base MediaHandler class

It was being called in File::getLength, but was only implemented
in a subclass, that's not even in core. Any method of a handler
class called from a method in File class should work on any
subclass of MediaHandler.

Change-Id: I94eda90ec3dd64b6ff2e3a5732aa539366cec521
Merge "Add default implementation of getLength to base MediaHandler class"