Make DataConnection VCN managed by default.

This CL makes DataConnection's (and their DcNetworkAgents) have the
Capability NOT_VCN_MANAGED by default. This fixes a bug where the
NOT_VCN_MANAGED bit was never set after calling to VcnManager (since it
is not a default capability).

Bug: 180781790
Test: atest DataConnectionTest
Change-Id: I259cbae4dd5506774175360de638f0416414add6