[FileRepo] [FileBackend] Added support for custom file headers.

[FileRepo] [FileBackend] Added support for custom file headers.

* For backends that support it, custom HTTP headers can be set on files.
* Added a getStreamHeaders() function to MediaHandler to let subclasses
  recommend header name/value pairs to be used for responses to GET/HEAD
  requests. For example, an OGG handler could set "X-Content-Duration".
* Made LocalFile use this function to set HTTP headers of new uploads.

Change-Id: I1b017e1342513f0097fe6d142aae18e819403293