Fixes two issue for build Vulkan backend for android ndk level < 26,
* Disable Vulkan validation layers for android ndk level < 26
* Share vulkan memory allocator implementation with chrome to avoid
  duplicated symbols link errors.
* Only run vulkan backend test with Android P or newer

Note: This change will break android-binary-size try bot, we need to
update expected_static_initializer_count to 4 at [1] while rolling this
change into chromium.

    [1] https://source.chromium.org/chromium/chromium/src/+/master:chrome/android/static_initializers.gni;l=19?q=expected_static_initializer_count&ss=chromium%2Fchromium%2Fsrc

Bug: chromium:1170339
Change-Id: Idb9238d8f339724c4d8f9ac136305b95ff06fae4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2656980
Commit-Queue: Peng Huang <penghuang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>