fastbootd: Add support for flashing logical partitions.

When flashing logical partitions, we read the "super" partition metadata
corresponding to the current slot. We then temporarily create a
device-mapper device for that partition, and immediately destroy the
device after all operations are complete. We do not mount partitions
ahead of time, or keep them mounted, because a fastboot operation may
change the layout of the logical partition table (or change which slot
is current).

Bug: 78793464
Test: fastboot flash a logical partition under "super"
Change-Id: Id2a61d3592decabeebfd283c4fd6e6cbe576a18c