Re-enable boot HAL tests and make sure they restore previous slot flags.

Bug: 154667790
Test: set slot to a
      run SetActiveBootSlot/SetSlotAsUnbootable tests
      fastboot reboot bootloader
      fastboot getvar current-slot shoud be a
      repeat on b slot
Change-Id: Icff45d3949d0ad76d22bdf8830c0903e81027b6e
Merged-In: Icff45d3949d0ad76d22bdf8830c0903e81027b6e