libcutils: android_reboot: add thermal shutdown command

Add ANDROID_RB_THERMOFF command to android_reboot(), denoting a power
off due to thermal limits exceeded.  When this command is used, Android
may provide notification to the user about the potentially unexpected
power off at next boot.

Bug: 30994946
Test: Manual: setprop sys.powerctl thermal-shutdown
Change-Id: If2cde1b6a3c281a5750b50295412a4801ed5b2f2