bootstat: Fix false metrics due to soft reboots.

Use a flag to enable/disable logging metrics, set to false during soft
reboots.

Bug: 32807863
Change-Id: Ib1359327af4e18d40400c29961c454f7be1b8d0c
Test: None
(cherry picked from commit 800ceb45b15c2b63edd4c699862653c55fe55ef2)