[Android] update ci.

Author: alienzh

.github/ci/build/build_android.sh

@@ -51,7 +51,7 @@ echo pwd: `pwd`
 echo sdk_url: $sdk_url
 unzip_name=Agora_Native_SDK_for_Android_FULL_DEFAULT
 zip_name=Agora_Native_SDK_for_Android_FULL_DEFAULT.zip
-if [ -z "$sdk_url" ]; then
+if [ -z "$sdk_url" ] || [ "$sdk_url" = 'none' ]; then
    echo "sdk_url is empty"
    echo unzip_name: $unzip_name 
    echo zip_name: $zip_name