Update DC mUnmeteredOverride in DcActiveState

Needs to update value so getNetworkCapabilities() correctly evaluates
meteredness
Moved NetworkAgent-dependent code from DcDefaultState to DcActiveState

Test: atest DcTrackerTest, DataConnectionTest, SubscriptionManagerTest
Bug: 150427710
Bug: 150417209
Change-Id: I01b7496334bd1fc8274d0c6a2de6cbc39f3f4931