Fork skia_vulkan_sdk default value on is_android.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4205

Change-Id: I091801af7e5e0093ef01a5708c0994928997ddf3
Reviewed-on: https://skia-review.googlesource.com/4205
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index dcc61dd..f5ad42f 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -32,8 +32,11 @@
 }
 
 declare_args() {
-  skia_use_vulkan =
-      skia_vulkan_sdk != "" || (is_android && defined(ndk_api) && ndk_api >= 24)
+  if (is_android) {
+    skia_use_vulkan = defined(ndk_api) && ndk_api >= 24
+  } else {
+    skia_use_vulkan = skia_vulkan_sdk != ""
+  }
 }
 
 # Our tools require static linking (they use non-exported symbols).