fastboot: Add getvar commands to query battery part info.

This adds two commands to test the new v3 Health HAL, to query the
battery serial number (unlocked devices only) and part status.

Bug: 309792384
Test: adb reboot fastboot
      fastboot getvar battery-part-status
      fastboot getvar battery-serial-number
Change-Id: Ie0a14eda06483d047544833124c327fb88ba43a2