only copy NDK binaries when using NDK in our standalone builds

  i.e. not when generating Android.bp (and in the future, not in Clank builds)

BUG=skia:6065

Change-Id: I52bdd5f01deb5e5b4d9e72bec197a82dc887be88
Reviewed-on: https://skia-review.googlesource.com/6941
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 90e8ffd..e214251 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1413,7 +1413,7 @@
     }
   }
 
-  if (is_android) {
+  if (is_android && is_skia_standalone && ndk != "") {
     copy("gdbserver") {
       sources = [
         "$ndk/$ndk_gdbserver",