Added dialog to notify user of thermal shutdown

tldr; Have the phone check when it reboots if it last shut
down due to heat. If so, we should show the user an
notification and dialog giving them more details.

- Added hidden api to allow apps to query system_server
for the reason the phone last rebooted
- Added notification that is shown when the phone is
booted if the last shutdown was due to excessive
heat.
- Added dialog to provide more details about the
shutdown if the notification is tapped.
- Added tests to verify that file is read and
as expected.
- Updated code for high temperature warning that
is shown while the phone is running to avoid
mixups in the future.

Test: FrameworksServiceTests
Bug: 30994946
Change-Id: Ic25f42539911c89ba7f1834e206f7931d65c2865