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
fb95e121d8