Add a module-lib API for constructing a LocalSocket from an fd.

This allows constructing a LocalSocket from a FileDescriptor
referring to an already-connected socket. The LocalSocket can
then be used to exchange ancillary filedescriptors and fetch
peer socket credentials for the existing FD.

The fd is not dup'd or closed by the API, and the caller is
responsible for closing it. This is consistent with the
constructor of the related LocalServerSocket class, which also
takes an fd and does not manage its lifecycle.

Bug: 200200870
Test: atest CoreTests:android.net.LocalSocketTest CtsNetTestCases:android.net.cts.LocalSocketTest
Change-Id: I86448ba80475c4563194a4a2d9a0c0bbd0b76444