Switch fastboot to new A/B spec

Slots are now referred to as a and b instead
of _a and _b. For the moment, _a and _b will
still be supported. For old devices that
support A/B, they are assumed to have 2 slots.

Bug: 29643845
Change-Id: Ieab6bed7b4977d8386dcec0afa434371f54bd63f
(cherry-picked from commit 563fcf6f)