Use screen dimensions as default video size

Instead of 720p, use the screen dimensions as the default video
size.  For example, a phone with a 768x1280 display would be
recorded as a 768x1280 video when in portrait, or a 1280x768
video when in landscape.

If the device is not able to record at that resolution, we
fall back to 720p.

Also, fixed a problem introduced by a buffer timestamp change.

Change-Id: I7230ddb323c669ab07327f26df8d103c192c21d3