vendor: Add chromium prebuilt support (2/2)

Ported from https://github.com/MoKee/android_vendor_mk/commit/888c49320320b771466b5cfe9e494108aa267e65
export USE_PREBUILT_CHROMIUM=1 to enable

Change-Id: Ide9f31f3ec2769c015121dbc12ece5e14445971c