libstagefright: Filewrite in seperate thread for single video track

For files with single video track, the filewrite() and OMXRead()
operations are happening in the samethread. It impacts the
recording performance.

Hence moving filewrite to seperate thread for files with single
video track.

CRs-Fixed: 574807

Change-Id: I3a9bf7869e197973a85783510254cf66b8d1d91b