set_active command should update current slot information.

Boot control HAL implementations will return the current boot
slot as the active slot. If a set_active command is issued on another
slot, it should be considered as the new active slot and subsequent
flashes should flash the same.

Test: fastboot set_active "b", fastboot getvar current-slot
Bug: 78793464
Change-Id: Ida3817670de8e74a7d7ae2a905e7ac1756c6bdf1