fastboot: Use a single codepath for flashall and update.

This patch extracts the common logic out of the flashall and update
command implementations. There is now a FlashAllTool helper class, and
an ImageSource class for commands to specify how to find built images.

With these paths merged, the update command now supports logical
partitions.

Bug: 78793464
Test: flashall works with or without a super partition
      update works with or without a super partition

Change-Id: I63a8690bbc4da6ea98a07eb2c07166ddd993a7b7