Make IaPrefixOption constructor public.

Then we can create an IaPrefixOption instance via constructor in the
NetworkStackIntegration test.

Bug: 260934173
Test: atest NetworkStackIntegrationTests
Change-Id: I05be1dde6ba5bda125631674c5c0f8634eb18924
diff --git a/staticlibs/device/com/android/net/module/util/structs/IaPrefixOption.java b/staticlibs/device/com/android/net/module/util/structs/IaPrefixOption.java
index 060d491..f0e4409 100644
--- a/staticlibs/device/com/android/net/module/util/structs/IaPrefixOption.java
+++ b/staticlibs/device/com/android/net/module/util/structs/IaPrefixOption.java
@@ -18,6 +18,8 @@
 
 import static com.android.net.module.util.NetworkStackConstants.DHCP6_OPTION_IAPREFIX;
 
+import androidx.annotation.VisibleForTesting;
+
 import com.android.net.module.util.Struct;
 import com.android.net.module.util.Struct.Field;
 import com.android.net.module.util.Struct.Type;
@@ -67,7 +69,8 @@
     @Field(order = 5, type = Type.ByteArray, arraysize = 16)
     public final byte[] prefix;
 
-    IaPrefixOption(final short code, final short length, final long preferred,
+    @VisibleForTesting
+    public IaPrefixOption(final short code, final short length, final long preferred,
             final long valid, final byte prefixLen, final byte[] prefix) {
         this.code = code;
         this.length = length;