Better FileProvider API

[GoogleCodeExporter]

The current FileProvider API is bad. I recently tried to implement 
this library in another software, but with the current API it's not 
possible to read from other sources than files (buffers, network etc).

There was a workaround before, which involved passing the filename as
the context, but this doesn't work since the mp4v2 tries to read the
file size. 

There has been some talk about it here
https://groups.google.com/forum/#!searchin/mp4v2/readprovider/mp4v2/ZNbXTgb6dkI/
NhhCAnIN_yAJ
but as far as I can see nothing has happened.

I wouldn't mind looking into it, but it'd be good to know if you have
any plans for it already.

Original issue reported on code.google.com by [email protected] on 4 Apr 2014 at 12:37