Current number of threads selection code does not work well for Android builds - middle and low end devices are having hard time encoding VGA and QVGA with just one thread. Increase the amount of vp8 encoder threads for 180p and above resolution. Also limit maximum number of thread to 3, since for 8 core devices most of time 4 cores are idle when thermal throttling kicks in. BUG=b/27946721 R=marpan@webrtc.org Review URL: https://codereview.webrtc.org/2058753003 . Cr-Commit-Position: refs/heads/master@{#13142}