Fix checkNotNull error for missing Vpn service in Wear

The VpnManagerService does not work today on Wear devices, so we stopped
starting it in ag/28141663. However, the VpnManager has a checkNotNull
assertion when it tries to access the service. That means, all Wear
callers of Context.getSystemService for VpnManager will experience a
crash since the service is null on Wear.

To fix this, we skip the checkNotNull check on Wear devices by throwing
a ServiceNotFound exception from SystemServiceRegistry. That way, the
registry also knows that the service is not available on the watch
device and avoid retrying the fetch for the service.

Flag: android.server.allow_removing_vpn_service
Test: fetching VpnManager on watch returns null with no crash
Bug: 352607724
Bug: 352607425
Bug: 352614765
Bug: 340928692
Change-Id: I5e554cb082ab4f687ae6755c9ee52fc8372e9bd8