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
4 files changed