Cap pts gap between adjacent frames to specified value

- In the scenario of cast mirroring, encoding could be suspended
  for prolonged periods. Limiting the pts gap to workaround the
  problem where encoder's rate control logic produces huge frames
  after a long period of suspension.

- Repeat last frame a couple more times to get better quality
  on static scenes.

- Fix the timestamp on repeat frames (it was not set)

Bug: 11971963
Change-Id: I1d68ab3d269874bf3921aa429a985c5f63e428c7