Add a new PIO flag: DHCPv6-PD preferred flag.

The new P flag is defined in
https://datatracker.ietf.org/doc/draft-ietf-6man-pio-pflag/.

Bug: 315069348
Test: TH
Change-Id: Ie386cc8b9fbe136333b46c73da785e02983a582b
diff --git a/staticlibs/framework/com/android/net/module/util/NetworkStackConstants.java b/staticlibs/framework/com/android/net/module/util/NetworkStackConstants.java
index f9895c6..7c4abe0 100644
--- a/staticlibs/framework/com/android/net/module/util/NetworkStackConstants.java
+++ b/staticlibs/framework/com/android/net/module/util/NetworkStackConstants.java
@@ -189,6 +189,7 @@
 
     public static final byte PIO_FLAG_ON_LINK = (byte) (1 << 7);
     public static final byte PIO_FLAG_AUTONOMOUS = (byte) (1 << 6);
+    public static final byte PIO_FLAG_DHCPV6_PD_PREFERRED = (byte) (1 << 4);
 
     /**
      * TCP constants.