Fix the media memeory stress test.

The media memory stress test used the ps mediaserver to
capture the memory usage of mediaserver. As the media.log
is enabled recently, when runnign the ps mediaserver, it will
show both entries. The fix is simply skip the first two lines
and parse the laste output.

Change-Id: Ib2a4ba7936505b4ea24463bf8a5af2c499b71138