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

tests/common/java/android/net/IpPrefixTest.java

@@ -122,6 +122,9 @@ public class IpPrefixTest {
 
         p = new IpPrefix("[2001:db8::123]/64");
         assertEquals("2001:db8::/64", p.toString());
+
+        p = new IpPrefix(InetAddresses.parseNumericAddress("::128"), 64);
+        assertEquals("::/64", p.toString());
     }
 
     @Test