bootstat: reboot cold, warm and hard tests failed

If the bootloader insists on reporting reboot for cold, warm and
hard, we need to reconstruct canonical reason from the
persist.sys.boot.reason property.

Some log lines contained bootstate, letting their noise through
during testing, changed regex to look for bootstat[^e].

Test: system/core/bootstat/boot_reason_test.sh
Bug: 63736262
Change-Id: I3788c6cf8aca7fc73dd01bf95acb596d18ed7ee4