Stop building GL backend on Android Vulkan builders

Should reduce build time and avoid running GL unit tests redundantly
on Vulkan bots.

Change-Id: I67379e6820dab2f038175a825ee1461086a2da84

Cq-Include-Trybots: luci.skia.skia.primary:Test-Android-Clang-Pixel4XL-GPU-Adreno640-arm64-Release-All-Android_Vulkan
Change-Id: I67379e6820dab2f038175a825ee1461086a2da84
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/318196
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
3 files changed