ADD new enum NetworkType to identify multiple transport type in

stats.proto

Add new NetworkType for multiple transport
1. CELLULAR + VPN = NT_CELLULAR_VPN
2. WIFI + VPN = NT_WIFI_VPN
3. BLUETOOTH + VPN = NT_BLUETOOTH_VPN
4. Ethernet + VPN = NT_ETHERNET_VPN

Bug: 143732914
Test: atest
Change-Id: I24ceff6bc95af79d92d3e39695b24d10763cdb4e