Rearrange how AF_UNIX sockets are handled for accept()

Contains the following changes:
1) Introduction of UnixSocketAddress / removal of InetUnixAddress.
2) Os/Posix classes now take SocketAddress instead of InetSocketAddress.
3) Removal of native code that dealt with InetUnixAddress.
4) Introduction of OsConstants.UNIX_PATH_MAX to hold the maximum sun_path
5) Update tests.

Bug: 3106438
Change-Id: I47a3618387f21599bfbfd0dc9a821839bf47b39b
13 files changed