fastbootd: Support two super partitions for retrofit devices.

Retrofit devices will have two super partitions, spanning the A and B
slots separately. By design an OTA will never cause "A" or "B"
partitions to be assigned to the wrong super. However, the same is not
true of fastbootd, where it is possible to flash the inactive slot. We
do not want, for example, logical "system_a" flashing to super_b.

When interacting with partitions, fastbootd now extracts the slot suffix
from a GetSuperSlotSuffix() helper. On retrofit devices, if the partition
name has a slot, that slot will override FastbootDevice::GetCurrentSlot.
This forces partitions in the inactive slot to be assigned to the correct
super.

There are two consequences of this. First, partitions with no slot
suffix will default to the current slot. That means it is possible to
wind up with two "scratch" partitions, if "adb remount" is used on both
the "A" and "B" slots. However, only the active slot's "scratch" will be
visible to the user (either through adb or fastboot).

Second, if one slot does not have dynamic partitions, flashing will
default to fixed partitions. For example, if the A slot is logical and B
is not, flashing "system_a" will be logical and "system_b" will be
fixed. This works no matter which slot is active. We do not try to
upgrade the inactive slot to dynamic partitions.

Bug: 116802789
Test: fastboot set_active a
      fastboot flashall # dynamic partitions
      fastboot getvar is-logical:system_a # true
      fastboot getvar is-logical:system_b # false
      fastboot set_active b
      fastboot flashall --skip-secondary
      fastboot getvar is-logical:system_a # true
      fastboot getvar is-logical:system_b # true
      Booting both slots works.

Change-Id: Ib3c91944aaee1a96b2f5ad69c90e215bd6c5a2e8