Fix: public_android_sdk not defined when linux toolchain used.

In build/util/version.gni, `public_android_sdk` is used when
`target_os == "android"`.
But in build/config/android/config.gni, it's defined only when
`is_android || is_chromeos`.

This PR fixes `public_android_sdk` not defined error when
`target_os == "android"` and host toolchain (linux) used.

Change-Id: I41f75ac629ea5009a5e1e404918d256be0cbe77d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2096799Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Richard He <linyhe@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#749155}

build/util/version.gni

@@ -56,7 +56,7 @@ if (is_mac) {
     target_cpu,
   ]
 
-  if (!public_android_sdk) {
+  if (defined(public_android_sdk) && !public_android_sdk) {
     _script_arguments += [ "--next" ]
   }
 }