PowerUI: Update temperature warnings on thermal throttling events

Subscribe to thermal throttling start/stop events from thermalservice,
if the vendor thermal management subsystem supports push notifications
of throttling events.  Update temperature warnings immediately when
throttling start/stop events are received.

Fixup existing comment to reflect current code regarding default warning
temperature value.

Bug: 66698613
Test: PowerNotificationWarningsTest, PowerUITest,
      manual: taimen with artificially low temperature threshold and
      logcat debugging messages

Change-Id: I021bcd0d06af861fd3a18a1790815a836659cee6