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
diff --git a/framework/src/android/net/LinkAddress.java b/framework/src/android/net/LinkAddress.java
index 44d25a1..d1bdaa0 100644
--- a/framework/src/android/net/LinkAddress.java
+++ b/framework/src/android/net/LinkAddress.java
@@ -349,7 +349,7 @@
      * @return {@code true} if both objects are equal, {@code false} otherwise.
      */
     @Override
-    public boolean equals(Object obj) {
+    public boolean equals(@Nullable Object obj) {
         if (!(obj instanceof LinkAddress)) {
             return false;
         }