#gpowerd Track actions that are still validating during startup

* In MonitorAction, when an action is still validating,
  return an unavailable error instead of a not found error
* Adds a validating_actions_ set to the ActionContextManager
  and updates loading actions into the context manager accordingly

PiperOrigin-RevId: 891824774
Change-Id: I4ab2e1ca492cfcc04052c1fb79484058f120454b
3 files changed