Unhide IpPrefix(InetAddress, int)

Unhide IpPrefix constructor to allow its usage with the new
VpnService.Builder#excludeRoute(IpPrefix) method.

Bug: 186082280
Test: atest IpPrefixTest
Change-Id: Ie5b62b2b206def1be53a41219681b4a8bc06c1d2
diff --git a/framework/api/current.txt b/framework/api/current.txt
index c53f29a..a755234 100644
--- a/framework/api/current.txt
+++ b/framework/api/current.txt
@@ -206,6 +206,7 @@
   }
 
   public final class IpPrefix implements android.os.Parcelable {
+    ctor public IpPrefix(@NonNull java.net.InetAddress, @IntRange(from=0, to=128) int);
     method public boolean contains(@NonNull java.net.InetAddress);
     method public int describeContents();
     method @NonNull public java.net.InetAddress getAddress();