Return null instead of throwing ServiceNotFoundException when...

Return null instead of throwing ServiceNotFoundException when VPN_MANAGEMENT_SERVICE is not available.

This is to fix the issue that the system service registry throws an exception when the service is not available. There's a wtf log that happens if the UID of the app is a system UID.
Related CL: ag/28292869

Bug: 359924233
Change-Id: I041beac94395e4213443a026e5dbb038a09fa5c8
Test: manual
Flag: android.server.allow_removing_vpn_service