Add AttributionSource stub

The class was not present in older Android SDKs.

server/src/main/stubs/android/content/AttributionSource.java

@@ -0,0 +1,15 @@
+package android.content;
+
+public class AttributionSource {
+    public static class Builder {
+        public Builder(int uid) {
+            throw new UnsupportedOperationException();
+        }
+        public Builder setPackageName(String value) {
+            throw new UnsupportedOperationException();
+        }
+        public AttributionSource build() {
+            throw new UnsupportedOperationException();
+        }
+    }
+}