bootstat: better validation of battery level (shutdown,battery)

Replace simple strtoull with loop that ensures no leading zeros.
Restrict size of value buffer being checked as allocation was
going to end of retrieved buffer, which can cause unnecessary
memory pressure during boot.

Test: system/core/bootstat/boot_reason_test.sh
Bug: 63736262
Change-Id: Ifdc1d4fd3a73794c001577024ce7cbfde9c25028