Don't cause infinite loop if windows manager crashes in safe mode

We set ro.sys.safemode=1 on startup in windows manager in safe
mode so that safemode will be carried over to the second bounce for
fde (see b/28008032). However, if windows manager restarts for any
reason, it cannot set this value twice (read only) and throws.

Don't set if already set.

Bug: 79608610
Test: Kill Windows Manager in safe mode. Before this change, windows
      manager would reboot indefinitely. After this change, system
      comes back up quickly

Change-Id: I05f3adfa801eef0e5a1357a19fb7a3bf61ce02eb