Clear the warm_reset flag after boot is successful

The property is set to inform kernel to do a warm_reset on the next
reboot. This is useful to persist the logs to debug device boot
failures. After the slot has been marked as boot successful, we can drop
the warm_reset flag to avoid the performance overhead on the next
reboot.

Bug: 143489994
Test: check the property is set to 0 by update_verifier
Change-Id: I722fb1906e6efa56dfc4ad7beccd5e2ba7e0ef7c