Android: Use a file descriptor for all local media files.

Using a fd is more consistent across different Android versions and
also works with files that are in the applications private storage.

Task-number: QTBUG-39346
Change-Id: I462822459d12d7842d15f1cb7caafc75c18fe32c
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>