Calculate FRAME-RATE from sample duration as in the DASH solution.
Right now calculated frame-rate may not be accurate in some scenarios,
so we avoid setting the attribute in HLS unless it is more than 30
fps. We will set it unconditionally once it is fixed.
Fixes#634.
Change-Id: I87b6e9a047d959ae88dd4dcb2b4786527ba5c9fc