Revert "blank_screen: always exit with _exit()."

This was a temporary workaround in Android P, but it is fixed
with a relatively more dangerous change in libhwbinder.

This reverts commit 73ba34f1.

Bug: 77934844
Test: reboot device and watch for screen to get shutdown, check logs

Change-Id: I970144eea4acd8254f4a67ebe803f3c3eda9aba2