fastboot: Fix flashing both slots with dynamic partitions.

When updating the super partition, attempt to preserve partitions from
the other slot. If any partition can't be preserved, fail and require a
wipe (-w) to proceed. This allows two bootable builds to be flashed to
both slots.

The preserve operation can fail if the metadata is not compatible with
the old partition layout. For example, if the partition references a
group that no longer exists, or a group changed its capacity, or the
metadata's block device list or list contents changed.

Bug: N/A
Test: liblp_test gtest
      fastboot flashall --skip-secondary

Change-Id: I53fdd29bc1f0ef132005a93d3cf1cdcd7f2fc05f