AP_BattMonitor: Keep mavlink_charge_state instanced

[Georacer]

Summary

Currently the method will report unhealthy in BATTERY_STATUS.charge_state for ALL batteries, if one battery is unhealthy.

Fix it to refer to the current instance only.

Classification & Testing (check all that apply and add your own)

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

I did a SITL flight with a Loweheiser generator and 3 battery monitors configured:

BATT_MONITOR     4.000000 # Analog Voltage and Current
BATT2_MONITOR    17.000000 # Generator-Elec
BATT3_MONITOR    18.000000 # Generator-Fuel

Mid-flight, I turned off and on SIM_EFI_TYPE from 2 to 0.

Description

Currently the instanced MAVLink message BATTERY_STATUS.charge_state will go unhealthy for all backends.

This PR makes it so that it responds individually and correctly per-instance.