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
Merged-In: I695bc3f0e801bf13bc4fc0706565758f12b775b4
Merged-In: I6d3f3c50eaec44e3a0787e849ab28e89f6f4a72d
Merged-In: Iddfec82a08f845e728adadfa6ec58a60a078d6af
Merged-In: I8d6dd5efd226a8b1c4b05d1e1102362b58e094a1
Merged-In: Ied0cc53ac34c7c5f5539507b1979cbf9c215262e
Merged-In: I3b2b7dcb1a9a194fc08643b27bbb5a0e84e01412

(cherry picked from commit 1dfb6b67)

Change-Id: I9a17094bfdc54b9dec671306618e132a4beb59fc