Use ArraySet#add() instead of ArraySet#append()

ArraySet#append() is a hidden API which is not accessible for
mainline module, use public one - ArraySet#add() instead.

Bug: 170598012
Test: atest FrameworksNetTests
Change-Id: I0742e2ec7aff008141b1de6d10eeca2910df71b1

framework/src/android/net/NetworkCapabilities.java

@@ -1864,7 +1864,7 @@ public final class NetworkCapabilities implements Parcelable {
                 final ArraySet<T> result = new ArraySet<>(size);
                 for (int i = 0; i < size; i++) {
                     final T value = in.readParcelable(loader);
-                    result.append(value);
+                    result.add(value);
                 }
                 return result;
             }