fastbootd: use FastbootDevice::GetCurrentSlot to get current slot suffix

Using FastbootDevice::GetCurrentSlot to get the current slot suffix
instead of reading the same from the device's boot control HAL
is required to account for the prior set_active commands issued via
fastbootd in the same boot.

Test: fuzzy_fastboot
--gtest_filter=Conformance.Slots:Conformance.SetActive --gtest_repeat=10
Bug: 146589281

Change-Id: I2edb0f024d93c2483659623423ef5c69c717c7af
diff --git a/fastboot/device/commands.cpp b/fastboot/device/commands.cpp
index 2c9dec9..ca120c6 100644
--- a/fastboot/device/commands.cpp
+++ b/fastboot/device/commands.cpp
@@ -261,7 +261,7 @@
     }
 
     // If the slot is not changing, do nothing.
-    if (slot == boot_control_hal->getCurrentSlot()) {
+    if (args[1] == device->GetCurrentSlot()) {
         return device->WriteOkay("");
     }