Moved DcNetworkAgent out of DataConnection

DcNetworkAgent is a large class so moved it out to a
separated file. No logic change.

Test: Manual + unit test
Bug: 130299990

Merged-In: Ic1d610c79f5a91a154bf2a427dd2ebd10249dca7
Change-Id: Ic1d610c79f5a91a154bf2a427dd2ebd10249dca7
(cherry picked from commit 3ef02277)