Defer marking boot successful when checkpointing

This moves actually marking the slot as successful to a later point
so that on devices with checkpointing enabled we can still roll back to
the previous version if we fail to boot to the point that the checkpoint
is marked as successful.

Test: When taking an update on a checkpoint enabled device, it
      defers marking the slot as successful instead of directly
      marking it. Visible in logs.
Bug: 123260515

Change-Id: I7ed3595c1b0904ddbfe20d1cad4f69ecbf1ea351