add default implementation of Calls interface on Android KT target - For Unit tests mocking reasons

Author: danielpsantiago

src/target/kt_android.cr

@@ -61,7 +61,9 @@ END
         "callback: (error: Error?, result: Boolean?) -> Unit"
       end
       @io << ident(String.build do |io|
-        io << "   fun #{mangle op.pretty_name}(#{args.join(", ")}) \n"
+        io << "   fun #{mangle op.pretty_name}(#{args.join(", ")}) { \n"
+        io << "       callback(Error(ErrorType.Fatal, \"Not Implemented\"), null) \n"
+        io << "   }\n"
       end)
     end
     @io << <<-END

target-android/api/build.gradle

@@ -24,7 +24,7 @@ dependencies {
     implementation fileTree(dir: 'libs', include: ['*.jar'])
     api 'com.squareup.okhttp3:okhttp:3.10.0'
     api 'com.anupcowkur:reservoir:3.1.0'
-}
+}`
 
 publishing {
     repositories {