Only download Android SDK for android checkouts.

This should solve the bug where LUCI win bots try to download the SDK
and fails.

Bug: chromium:869249
Change-Id: I374ecf6508d6e3978559ec3251b89faaec99edd6
Reviewed-on: https://chromium-review.googlesource.com/1168500Reviewed-by: Ryan Tseng <hinoka@chromium.org>

DEPS

@@ -798,6 +798,7 @@ hooks = [
     # This downloads SDK extras and puts them in the
     # third_party/android_tools/sdk/extras directory.
     'name': 'sdkextras',
+    'condition': 'checkout_android',
     'pattern': '.',
     # When adding a new sdk extras package to download, add the package
     # directory and zip file to .gitignore in third_party/android_tools.