healthd: charger mode use reboot(), not android_reboot()

Preserve last Android reboot reason as recorded by android_reboot()
prior to entering charger mode.  For example, a thermal shutdown that
reboots to charger mode leaves that reason in place, for user
notification purposes at next Android startup.

Bug: 30994946
Test: manual
Change-Id: I003f57bfff29a471bb647d2cf6b0986ff428985a