Make checkpoint_gc.sh robust against interference

If some other process decides to run a brief period of GC and then turn
it off, checkpoint_gc.sh will keep running and hold up the OTA. To
combat that, we set gc_urgent again each cycle.

Test: set gc_urgent to 0 after the script sets it to 1. Verify script
      still runs

Change-Id: If361a7e6aace2b20d976d94210fc5a72c3630c72
1 file changed