Fix bugs in boot HAL VTS test

Remove copied code and ensure that changes to boot slots are rolled
back to prevent problems on reboot.

Bug: 34220303
Test: Tests pass; device reboots successfully afterward.
Change-Id: I40f8ec283beae3feb09ab6f416fd4e178b173b87
Signed-off-by: Connor O'Brien <connoro@google.com>