audio:mute duration at the beginning of the recording

-Sometimes shutter sound is getting recorded because
 current kAutoRampStartUs is not sufficient to mute the
 shutter sound

-Fix is to increase the kAutoRampStartUs to 500ms.

Change-Id: Id37b48c4bb7b5c903ef502f933eaff7be0ea566c
CRs-Fixed: 636491