liblp: Add a helper method for upgrading metadata on retrofit devices.

This adds a new MetadataBuilder constructor, NewForUpdate, that can be
used by update_engine to simplify upgrading metadata. It is safe to call
whether or not the device is a retrofit. If the metadata has block
devices assigned to a specific slot, and that slot matches the slot
suffix, it will ensure that an equivalent entry exists for the alternate
slot.

Thus, if the source slot is _a and the target slot is _b, and the
metadata has "system_a" as a block device but not "system_b", this will
automatically add "system_b" as a block device.

Bug: 116802789
Test: liblp_test gtest
Change-Id: Ie89d4dbf4c708b5705e658220227ebf33fcb1930