bootstat: shutdown reports reboot

Some devices report the following canonical boot reason for all
shutdown operations:

    reboot,kernel_power_off_charging__reboot_system

because shutdown switches to a charging kernel, and reboots into the
system when the user presses the power button.  Thus last kernel
messages arrives as:

    <0>.(0)[53:pmic_thread]reboot: Restarting system with command \
                       'kernel power off charging  reboot system'
     -> "shutdown" (w/o last boot reason)
     -> "shutdown,<subreason>" (w/last boot reason)

The reboot reason from that charging instance propagates as a
fortified boot reason blocking interpretation of the last boot reason
that manages shutdown canonical boot reason determination.  The fix
is to change reboot,kernel_power_off_charging__reboot_system to
shutdown, so that it is viewed as a blunt reason that can be
overridden by last boot reason.

We added the above boot reason to kBootReasonMap because the Bit
Error Handler can use it to reconstruct if there is any damage to
the last kernel messages content.  The sad thing is that the enum
will never propagate as we are filtering it out and reporting
"shutdown" instead.  Of course, we are now covered for a can not
happen.

Test: boot_reason_test.sh
Bug: 74595769
Bug: 63736262
Change-Id: I28987f0871af7d967cc4bbbffed43bd42349acdd