Remove the usage of hidden API and simplify the design

- ConnectivityManager will be a part of mainline module, so its
hidden API cannot be called by external caller.
- There is no need to listen onNetworkSuspended() and
onNetworkResumed() because if the network is suspended, then
NET_CAPABILITY_NOT_SUSPENDED will be removed and
onCapabilitiesChanged() will be called and the network capabilities
will be different from the previous one. And when the network is
resumed, NET_CAPABILITY_NOT_SUSPENDED will be added back to the
network capabilities. So the current design can be simplified.

Bug: 182963397
Test: atest FrameworksVcnTests
Change-Id: Ib8f8ea1c95503c946b8229670512af2db8be3165