audio: mute duration at the beginning of recording

-AudioSource mutes for predefined duration (kAutoRampStartUs)
 at the beginning of a recording. Current kAutoRampStartUs
 is not sufficient to mute the shutter sound click on
 B-Family devices. Sometime shutter sound is getting recorded
 with the current kAutoRampStartUs.

-Issue with calculating muteDuration from getOutputLatency is,
 more delay is getting added for Bt(~1 sec) which is really not
 required.

-Fix is to use constant mute delay which is appropriate

Change-Id: I695e9d71a53bba4c344414d36cf55f83fdadec5f
CRs-Fixed: 636491