bootstat: system_boot_reason does not handle reboot_<reason> case

Heuristic would translate reboot_<reason> to reboot._<reason>. Add
checking/squashing for leading comma (,) _or_ underscore (_).

Test: compile
Bug: 63736262
Change-Id: I7ef4930ec8c064a8151183ac08d292931a8584c8
1 file changed