liblp: Don't store BlockDeviceInfo separately in MetadataBuilder.

There's no reason to do this, since the fields are already in
LpMetadataGeometry. Removing this also simplifies multiple-block-device
support.

Bug: 116802789
Test: liblp_test gtest
Change-Id: Ib244a98fdd9d36c94a2dffd81bef68a1d5644ab9