Add Android DisplayVk and WindowSurfaceVk variants.
Build Vulkan backend and validation layers on Android
if toolchain uses required NDK API level.
Fix validation layers discovery to work on Android.

BUG=angleproject:2314
TEST=angle_end2end_tests builds and runs on Nexus 5X, 12 VULKAN tests pass

Change-Id: Iac2ec4ecd6470a7552f9f60c023ba1760aa090c5
Reviewed-on: https://chromium-review.googlesource.com/887797Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>