boot_control: Don't allow current_slot_ to be -1.

If the HAL is run on a non-A/B device, it does not fail gracefully; it
will segfault because the current_slot is -1 and methods do not protect
against this. Instead, have Init() fail if we can't determine the
current slot.

Bug: 130078382
Test: vts tests
Change-Id: I2d3094518a842b1a764427d2da9d46781a9f26da