This CL optimizes the Android capture NV12 -> I420 + scaling code. For
example, when the input is 1280x720 and we adapt to 640x360, this CL:
- Reduces conversion time from 3.37 ms to 1.46 ms.
- Reduces memory footprint by 1 MB.
BUG=webrtc:6319
Review-Url: https://codereview.webrtc.org/2317443003
Cr-Commit-Position: refs/heads/master@{#14167}