Add @Nullable to Object#equals()

This is a partial cherry-pick of change:
I5eedb571c9d78862115dfdc5dae1cf2a35343580

for connectivity classes.

Bug: 170883422
Test: m
Merged-In: I5eedb571c9d78862115dfdc5dae1cf2a35343580
Change-Id: I7dc661863b73f4198ddb4f3a1566583d0f07db3c

framework/src/android/net/LinkProperties.java

@@ -1613,7 +1613,7 @@ public final class LinkProperties implements Parcelable {
      * @return {@code true} if both objects are equal, {@code false} otherwise.
      */
     @Override
-    public boolean equals(Object obj) {
+    public boolean equals(@Nullable Object obj) {
         if (this == obj) return true;
 
         if (!(obj instanceof LinkProperties)) return false;