bootstat: handle a bad bit error rate issue with pstore

Create a private rfind that allows a fuzzy match based on a bit error
rate (BER) of 1 every 8 bits.  last kmsg is affected by pstore ramoops
backing that suffers from data corruption.  Add some additional
validation based on possible data corruption scenarios, as a noisy
match means higher chance of noisy data.

Noisy data notably can affect the battery level detection, but do not
typically result in false positives.  Battery level, or failure, is
the responsibility of the BatteryStats service, providing a positive
signal and strong device-independent algorithm.  The checking done in
bootstat is likely to be deprecated in favour of an API request to
BatteryStats once their algorithms deal with surprise outages due to
aging.

The kernel logging heuristic and BER fixup handily deals with a
prevalent issue where some bootloaders failure to properly notify us
of panics.  This is where the gains are noticed with this improvement.

Test: system/core/bootstat/boot_reason_test.sh
Bug: 63736262
Change-Id: I93b4210f12fb47c5c036f4d6eb4cafeee4896d35