Enables encoder for libvpx.

Based on libvpx v1.2.0 this change removes the
disable-encoder flags from all build configurations
thus enabling encoder and updates other build
files in accordance with UPDATING.

Change-Id: Ief7c74593fc4a585d4a8b1768a4e1008547f0c40
Signed-off-by: Kunter Gultekin <kuntergultekin@google.com>