Move TcpKeepalivePacketData out of the framework.

This class is not used in framework.jar, only in services.jar.
Move it out of the framework so framework.jar can stop depending
on the NetworkStack stable AIDL interfaces.

This should really under com.android.server as well, but that's
a cross-project change to be done in another CL.

Test: m
Bug: 128804404
Change-Id: Ib2805f7fcc516a5f6989c252365d1c95b8042703