Fix dead lock in Tethering state machine

This fix is for tethering from N MR1. It reserves the same idea
originally in e4bdfd8ad86a013c5a2ee8964119393cea1ec4a3.

Tethering is animated by two state machines. If an error occurs,
one of the state machine notifies the other and definitely stays
in the error state while the other retries the tether operation
after having treated the error.

An example is when dnsmasq is started by calling tether(), but while it
hasn't finished starting yet untether() is called asynchronously
by a different tethering. Then "Setting DNS forwarders failed!" error
occurs and device can't be tethered anymore without reboot.

This patch allows to clear the error of the blocked state machine
from the other one and thus recover a normal cycle.

Test: fastly switching USB/Wi-Fi tethering On/Off repeatedly
Test: manually kill dnsmasq during BT tethering then turn on
      Wi-Fi tethering

Change-Id: Ide062dd4c15a0e7c72eced9a5fe807f845d4ea9c