API for apps to tag sockets with their own UID.

This enables app A to create a socket, pass it to app B, and have
app B accept blame for the traffic performed on that socket.

Also adds helpful public APIs for tagging raw FileDescriptor sockets
instead of making developers go through shady SocketImpl wrappers.

Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.AppSecurityTests#testAppFailAccessPrivateData
Bug: 63932076
Change-Id: I08925c843974675fc82e4080cec2eaab9ab7cd41