Run VP8 HW only on M and above devices.
Exynos H.264 HW encoder does not use frame timestamps for
bitrate allocation, so bitrate target need to be adjusted based
on current camera frame rate.
BUG=b/28738766
R=magjed@webrtc.org
Review URL: https://codereview.webrtc.org/2004973003 .
Cr-Commit-Position: refs/heads/master@{#12900}