init: Do full shutdown even for thermal shutdown

- Skipping SIGTERM / SIGKILL / umount brings race between block
  device driver and fs layer. Do umount before shutting down.
- Reduce timeout to 1 sec for thermal shutdown and skip other time
  taking part like fsck.
- Refactor waiting part to check time in ms so that 1 sec can
  have enough resolution.

bug: 63686426
Test: adb shell setprop sys.powerctl thermal-shutdown, adb shell setprop sys.powerctl reboot and check dmesg
Merged-In: I048bac767b328c8d656a97fe65dde5f2b5bf4ae5
Change-Id: I048bac767b328c8d656a97fe65dde5f2b5bf4ae5