liblp: Modify NewForUpdate to accomodate two super partitions.

This method was designed for a single-super model, and now needs to
change to accomodate two super partitions (system_a and system_b, for
retrofitting).

NewForUpdate is supposed to transition metadata from one block device
to the next for updates. For normal devices this is a no-op, since
metadata only exists on one partition (super). For retrofit devices,
metadata exists on system_a and system_b. This has two implications.

First, any references to the source slot must be rewritten. For example
"vendor_b" must become "vendor_a". However this is not true of partition
names. Partitions/extents are cleared in the updated metadata since they
no longer have any meaning (the block device list has been
rewritten). We also clear groups since they are re-added during OTA.

The reason we have to do this rewriting is that slot suffixes are
automatically applied in ReadMetadata. We do not have access to the
original unsuffixed metadata that was written by the initial OTA.
This was a conscious design decision, since it localizes retrofitting
idiosyncracies to just a few places (ReadMetadata, NewForUpdate, and
fastbootd), minimizing the number of external callers that have to
understand auto-slot-suffixing.

It would be arguably cleaner if retrofit metadata was always serialized
*without* slot suffixes, thereby making NewForUpdate a no-op. However
this would necessitate changes to the API elsewhere. The functions that
read partition names would have to take a slot suffix, and this would
further complicate MetadataBuilder and fastbootd. Another solution would
be to augment LpMetadata to retain unsuffixed information, but this is
probably not worthwhile given that retrofitting is intended to be
surgical, and will have a shorter lifespan than the non-retrofit case.

Bug: 116802789
Test: liblp_test gtest
Change-Id: I33596d92b38c47bc70bc0aa37ed04f6f0b9d4b6f