Interface with health HAL to read battery voltage

Bug: 78793464
Test: fastboot getvar battery-voltage
Change-Id: Ie0763e4f08327ec7649c5629066eb73e8142e0e6
diff --git a/fastboot/device/commands.cpp b/fastboot/device/commands.cpp
index 4fa1148..b02d968 100644
--- a/fastboot/device/commands.cpp
+++ b/fastboot/device/commands.cpp
@@ -96,6 +96,7 @@
             {FB_VAR_IS_LOGICAL, {GetPartitionIsLogical, GetAllPartitionArgsWithSlot}},
             {FB_VAR_IS_USERSPACE, {GetIsUserspace, nullptr}},
             {FB_VAR_OFF_MODE_CHARGE_STATE, {GetOffModeChargeState, nullptr}},
+            {FB_VAR_BATTERY_VOLTAGE, {GetBatteryVoltage, nullptr}},
             {FB_VAR_HW_REVISION, {GetHardwareRevision, nullptr}}};
 
     if (args.size() < 2) {