liblp: Allow automatic slot suffixing of partition names.

On retrofit devices, an OTA package or super_empty.img won't know which
slot it applies to. This is not an issue on devices shipping with
dynamic partitions, since they ship on the "a" slot.

To work around this, partitions and block devices can be flagged as
"auto-slot-suffixed". This indicates that ReadMetadata should
automatically append a slot suffix before returning the metadata. This
flag is added by MetadataBuilder when requested, and will be enabled via
lpmake separately.

After ReadMetadata has applied slot suffixes, it takes care to remove
the slot-suffix flag. This prevents the suffix from being applied twice,
if for example the metadata is then imported into a MetadataBuilder.

Bug: 116802789
Test: liblp_test gtest
      retrofit device boots

Change-Id: Ic7de06d31253a8d5b8d15c0d936175ca2939f857