Add missing @addtogroup tags.

These NDK docs weren't in any groups, so they don't show up in the
new site.

Test: https://irina-dot-devsite.googleplex.com/ndk/reference/group/native-activity
Test: https://irina-dot-devsite.googleplex.com/ndk/reference/group/networking
Test: https://irina-dot-devsite.googleplex.com/ndk/reference/group/tracing
Bug: http://b/77236573
Change-Id: Ic3154f77de1f13fe753b4c03537eab41b0a22848
diff --git a/include/android/multinetwork.h b/include/android/multinetwork.h
index 97892f8..5f2a57f 100644
--- a/include/android/multinetwork.h
+++ b/include/android/multinetwork.h
@@ -14,6 +14,15 @@
  * limitations under the License.
  */
 
+/**
+ * @addtogroup Networking
+ * @{
+ */
+
+/**
+ * @file multinetwork.h
+ */
+
 #ifndef ANDROID_MULTINETWORK_H
 #define ANDROID_MULTINETWORK_H
 
@@ -104,3 +113,5 @@
 __END_DECLS
 
 #endif  // ANDROID_MULTINETWORK_H
+
+/** @} */