vpx_ios/VPXExample: Play at input frame rate.

Use input time stamps to control display of video frames instead
of rendering as fast as frames are decoded.

Change-Id: I9f3bc81adf0cb38e58ca9b9c33961abf88c723a1
vpx_ios/VPXExample: Rename vpx_test_common.h to vpx_example_common.h

Remove the last vestiges of the original app name, and hopefully make
the example code more clear for the intended audience.

Change-Id: I83ccd48b0d4a38252d1524ab62c75a3b446eabf3