GN: Android Vulkan support

Android's the easiest place to start supporting Vulkan, so it's up first.

ndk_api becomes user-specifiable so that the Vulkan bots can kick it up
to 24, the first release supporting Vulkan.  The defaults remain the same:
18 for 32-bit and 21 for 64-bit, the first release supporting 64-bit.

To test this, I set ndk_api=24 and skia_use_vulkan=true in GN, then
   $ ninja -C out dm; and droid out/dm --config vk --src gm

Seems to work!  Bot scripts to follow.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2336343002

Review-Url: https://codereview.chromium.org/2336343002
3 files changed