healthd: Add support for the new Health HAL v3 properties.

Add BATTERY_PROP_PART_STATUS support via getProperty.

Add serial number support via a new helper function, since it doesn't
fit neatly into the BatteryProperty structure.

There are no sysfs nodes for this yet, so currently it is stubbed.

Bug: 309792384
Test: fastboot getvar battery-part-status
      fastboot getvar battery-serial-number
Change-Id: I3777e092d4ef26c02a7dd5420c9eaf1ec5aeae92