Remove check that preferred lifetime >= t2

For a single address, DHCPv6 won't work well (i.e. potentially lose provisioning) if
the preferred lifetime is lower than t2. However, this might be a valid
scenario in a renumbering event in the presence of multiple prefixes.

Additionally, in DHCPv6 it is up to the server to configure the client
correctly.

Test: TH
Change-Id: I37b2a0b89deda85b3f7be29c8f02f685aa6e65f8

staticlibs/device/com/android/net/module/util/structs/IaPrefixOption.java

@@ -88,8 +88,10 @@ public class IaPrefixOption extends Struct {
 
     /**
      * Check whether or not IA Prefix option in IA_PD option is valid per RFC8415#section-21.22.
+     *
+     * Note: an expired prefix can still be valid.
      */
-    public boolean isValid(int t2) {
+    public boolean isValid() {
         if (preferred < 0 || valid < 0) {
             Log.w(TAG, "IA_PD option with invalid lifetime, preferred lifetime " + preferred
                     + ", valid lifetime " + valid);
@@ -105,11 +107,6 @@ public class IaPrefixOption extends Struct {
                     + " longer than 64");
             return false;
         }
-        // Either preferred lifetime or t2 might be 0 which is valid, then ignore it.
-        if (preferred != 0 && t2 != 0 && preferred < t2) {
-            Log.w(TAG, "preferred lifetime " + preferred + " is smaller than T2 " + t2);
-            return false;
-        }
         return true;
     }