If the Vulkan backend is available, use it on Android.
This way ANGLE built for the ANGLE apk always uses Vulkan.
ANGLE built for use in Chrome keeps GL. At least for now.

Bug=angleproject:2418

Change-Id: Ic7796c74821a44b856a596d31b3de09e7ca47ede
Reviewed-on: https://chromium-review.googlesource.com/c/1265504Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@google.com>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>