Move VPN code from ConnectivityService to VpnManagerService.

ConnectivityService itself does not depend on mVpns or the Vpn
class any more. Most of this CL is simply moving code from one
class to another:

- Move the AIDL calls from IConnectivityManager to IVpnManager.
- Move the implementation from ConnectivityService to
  the new VpnManagerService.
- Move the APIs from ConnectivityManager to VpnManager, but
  temporarily maintain some shims in ConnectivityManager for the
  moved calls so that existing callers do not have to be modified
  in this CL.
- Update VpnService to call IVpnManager instead of
  IConnectivityManager.
- Move the code that registers the VpnManager service from
  ConnectivityFrameworkInitializer to SystemServiceRegistry.

Bug: 173331190
Test: atest HostsideVpnTests FrameworksNetTests CtsNetTestCases
Change-Id: I4911e2144df721a94fa00da9edf0dc372a7091c2