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
Merged-In: I2edb0f024d93c2483659623423ef5c69c717c7af