blank_screen: always exit with _exit().

Delay starting the threadpool until necessary,
and once started, always exit with _exit().

Bug: 77934844
Test: screen is blanked on reboot
Change-Id: I7715d889a6f78b9ce058fb3841ef7944116d8e82

light/utils/main.cpp

@@ -55,5 +55,6 @@ int main() {
         }
     });
 
-    return 0;
+    // b/77934844: Avoid running static destructors.
+    _exit(1);
 }