Clear mInterface before calling resetIkeState()

Clear mInterface before calling resetIkeState() in
onDefaultNetworkChanged().
resetIkeState() will trigger interfaceRemoved() to be called.
If mInterface is set, interfaceRemoved() will clear
Ikev2VpnRunner which makes VPN disconnect.

This issue can be reproduced when device establishes VPN
connection with mobile data first then connects to wifi.
In this case, onLost() for mobile data will not be called
because there is a new network(wifi) can satisfy the request,
so only onAvailable() for wifi will be called.
Which means onSessionLost() will not be called and only
onDefaultNetworkChanged() will be called, which makes that
mInterface is not cleared before interfaceRemoved() is called.

Bug: 219546241
Test: Check if VPN is still there when establishing VPN with
      mobile data first, then connect to wifi and disconnect
      wifi.
Change-Id: I7f9a1d9afd2a40762e9fac68edf1fb8ae75df8bc