Have vpxenc use a default kf_max_rate of 5 seconds.

Have vpxenc use a default kf_max_rate of 5 seconds.

Rather than using the static default maximum keyframe spacing
provided by vpx_codec_enc_config_default() set the default
value to 5 times the frame rate. Five seconds is too long for
live streaming applications, but is a compromise between seek
efficiency and giving the encoder freedom to choose keyframe
locations.

The five second value is from James Zern's suggestion in
http://article.gmane.org/gmane.comp.multimedia.webm.user/2945

Change-Id: Ib7274dc248589c433c06e68ca07232e97f7ce17f