Remove default enabling of vulkan on android in gn.

This now requires explicit setting of skia_use_vulkan to enable the use
of native vulkan on a device.

Change-Id: I8dc8b1ba878f5d742f39022e9888c5969645e639
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/791758
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
6 files changed