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
diff --git a/fastboot/device/variables.h b/fastboot/device/variables.h
index 9a46786..99d1355 100644
--- a/fastboot/device/variables.h
+++ b/fastboot/device/variables.h
@@ -67,6 +67,10 @@
                    std::string* message);
 bool GetBatterySoCOk(FastbootDevice* device, const std::vector<std::string>& args,
                      std::string* message);
+bool GetBatterySerialNumber(FastbootDevice* device, const std::vector<std::string>& args,
+                            std::string* message);
+bool GetBatteryPartStatus(FastbootDevice* device, const std::vector<std::string>& args,
+                          std::string* message);
 bool GetSuperPartitionName(FastbootDevice* device, const std::vector<std::string>& args,
                            std::string* message);
 bool GetSnapshotUpdateStatus(FastbootDevice* device, const std::vector<std::string>& args,