bootstat: 3.18 kernel does not report "SysRq : Trigger a crash"

Use an alternate means to determine that the sysrq crash was
requested.  Also, to be CTS compliant, the kernel_panic subreason
must be in lower case.

Test: boot_reason_test.sh
Bug: 74595769
Bug: 63736262
Change-Id: Ica06960ce62d220a909006e365951376d672b7e6