GM: Resume Required Alert should be triggered off of ECM state (#1045)

* Resume Required Alert should be triggered off of ECM state

On GM cars the ECM goes into cruise state 4 (standstill) and in order
for the ECM to accept cruise inputs, the resume button must be pressed
to drive it to state 1 (active).

Previously rather than testing for if the ECM was in state 4, the wheel
speed sensors were checked that the vehicle was going less than 0.5 m/s.

In some cases this could cause the resume required alert to be displayed
before the car actually came to a stop as far as the ECM was concerned.
If the alert is triggered before state 4 and OP demands acceleration,
its possible for OP to acelerate while the Resume Required Alert is
displayed created a very confusing UX.

* After more review, the problem was actually with the 'if at_full_stop' check was inside an else statement preventing entering the stopped mode if braking is 0

old-commit-hash: 4b12aca941

selfdrive/car/gm/gmcan.py

@@ -67,8 +67,8 @@ def create_friction_brake_command(packer, bus, apply_brake, idx, near_stop, at_f
   else:
     mode = 0xa
 
-    if at_full_stop:
-      mode = 0xd
+  if at_full_stop:
+    mode = 0xd
     # TODO: this is to have GM bringing the car to complete stop,
     # but currently it conflicts with OP controls, so turned off.
     #elif near_stop: