improve state handling of obfsvpn

• improve state handling of obfsvpn
• make obfsvpns start method blocking so that we can either handle errors and abort a VPN connection attempt or wait until obfsvpn is in a running state before we start openvpn to connect to obfsvpn for transparent proxying the traffic
• try to restart obfsvpn in on different proxy port in case the default one is already bound

related to leap/android_libs/bitmask-core-android!12 (merged) and obfsvpn!79 (merged)

More generally this MR reduces the amount of cases where obfsvpn gets into a unusable state after changing settings such as locations or obfuscation settings while the VPN is running. In those cases sometimes the VPN constantly failed to re-connect and only killing and restarting the app resolved the issue.