Moved DcNetworkAgent into a static class

Now the network agent can be handovered to the new data connection
after handover, so we should break all the implicit references to
the outer class. Otherwise the agent cannot perform the right
action on the right data connection.

Test: Manual
Bug: 128453667
Change-Id: I8d170faf8892b11c6cc75b67460749f75bd260e1