Android: Make it possible for MediaPlayer to read from assets.

The Android MediaPlayer doesn't handle assets automatically, so we need to
open it and pass it in as a file descriptor.

Task-number: QTBUG-31422

Change-Id: I8ad4e20bc56fd42db693721fd186ef86c729195d
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>