Avoid copying for input recording frames in CameraSource

This is the part one.

- Let CameraSource be a MediaBufferObserver. It releases the recording frame when the ref count of a MediaBuffer containing
  the recording frame drops to 0.

  This reduces the CPU load from 90+% down to 50-60%.

Part two is related to the avoidance of copying the input video frames to the video encoder.
However, we are not able to use OMX_UseBuffer directly. Still work on the second part.

Change-Id: I906f1d054ae8bdcf82e1617f1fc120152f2eb2eb