Add missing nullability annotations.

To prepare for enabling MissingNullability Metalava check this CL
works on adding missing nullability issues that metalava flags if
we tell it to flag new things since API 29.

This is not a complete CL, mostly addresses public api and
toString/equals for @SystemApi

Exempt-From-Owner-Approval: Large scale nullability clean up
Bug: 124515653
Test: make -j checkapi
Merged-In: I109260842cfc25f06e40694997fcbb4afa02c867
Change-Id: I109260842cfc25f06e40694997fcbb4afa02c867

core/java/android/net/StaticIpConfiguration.java

@@ -236,6 +236,7 @@ public final class StaticIpConfiguration implements Parcelable {
         return lp;
     }
 
+    @NonNull
     @Override
     public String toString() {
         StringBuffer str = new StringBuffer();
@@ -267,7 +268,7 @@ public final class StaticIpConfiguration implements Parcelable {
     }
 
     @Override
-    public boolean equals(Object obj) {
+    public boolean equals(@Nullable Object obj) {
         if (this == obj) return true;
 
         if (!(obj instanceof StaticIpConfiguration)) return false;

core/java/android/net/apf/ApfCapabilities.java

@@ -17,6 +17,7 @@
 package android.net.apf;
 
 import android.annotation.NonNull;
+import android.annotation.Nullable;
 import android.annotation.SystemApi;
 import android.annotation.TestApi;
 import android.content.res.Resources;
@@ -91,6 +92,7 @@ public final class ApfCapabilities implements Parcelable {
         }
     };
 
+    @NonNull
     @Override
     public String toString() {
         return String.format("%s{version: %d, maxSize: %d, format: %d}", getClass().getSimpleName(),
@@ -98,7 +100,7 @@ public final class ApfCapabilities implements Parcelable {
     }
 
     @Override
-    public boolean equals(Object obj) {
+    public boolean equals(@Nullable Object obj) {
         if (!(obj instanceof  ApfCapabilities)) return false;
         final ApfCapabilities other = (ApfCapabilities) obj;
         return apfVersionSupported == other.apfVersionSupported