Add convenience methods to IpPrefix and LinkAddress

Also moving relevant test files into tests/net as part of runtest
framworks-net.

Also removes testHashCode in LinkAddress() because this test relies on
the assumption that hashCode() is stable across releases or jdk
versions, which is absolutely not true.

This creates maintenance work for little benefit since hashCode is
already tested as part of the equality test.

For instance this test is now broken because hashing for InetAddress
changed.

Bug: 62988545
Bug: 62918393
Test: runtest frameworks-net, added coverage in tests
Change-Id: I695bc3f0e801bf13bc4fc0706565758f12b775b4