light service: mark as shutdown critical

Required because the lights need to be turned off late in boot
(e.x. during fsck for disk health), and before, blank_screen
accessing light encountered a race condition where if it didn't
get scheduled before all of the processes which were getting
shutdown were getting killed, the screen wouldn't blank.

Bug: 74976325
Test: adjusting brightness continues to work
Test: screen blanks during shutdown at the correct time
    including when an artificial 5s delay is inserted when
    the phone normally fscks (which can also be quick)

Change-Id: I57ebc11d7ecf57ee327c681455712f4b53de6fd3
Merged-In: I57ebc11d7ecf57ee327c681455712f4b53de6fd3