Add testVpnTypesEqual to verify consistency

VPN types are defined in both VpnManager.java and NativeVpnType.aidl.
The definitions on both sides should match (except TYPE_VPN_NONE).

VpnManager.java:
TYPE_VPN_NONE = -1
TYPE_VPN_SERVICE = 1
TYPE_VPN_PLATFORM = 2
TYPE_VPN_LEGACY = 3
TYPE_VPN_OEM = 4

NativeVpnType.aidl:
SERVICE = 1
PLATFORM = 2
LEGACY = 3
OEM = 4

Bug: N/A
Test: atest android.net.VpnManagerTest#testVpnTypesEqual
Change-Id: Ie618e227d861100c5318da696140e486af1093a0