dashplayer: Memoryleak fix and fps calculation during buffering

1. mTrackName was allocated with a new call but was never
   deleted, due to which there is a memory leak.
2. Skip fps stats for accumulated frames after buffering as the
   buffering duration reduces the fps value.

Change-Id: Ice0a577e0a6a662b41b7b9e73b38f9854858acf1