Initialize the start time if not specified in meta data

If the start time is not specified in meta data, the start time
will be assigned with latency of AudioRecord, it's calculated by
allocated frame count which is not the real start time when the
first frame received

Initialize the start time to the current system time if not
specified in meta data

CRs-Fixed: 812379

Change-Id: Ia007ca4592fe1f96e105d4ec48305c578b677bb1