Gallery2: Fix crash for unsupported format on mute operation

MPEG4Writer does not support formats other than mp4/h264/h263. The
app needs to check for unsupported formats before starting any mute
operation. Add a condition in the app to check for unsupported
formats and display a toast message.

CRs-Fixed: 514347 696137
Change-Id: Ib2ce48d2b8ebaef80936bc7a446e1c97e3a10c31