liblp: Fix flashing metadata to multiple block devices.

Although metadata is only stored on the first block device, we ideally
want to validate all the block devices before flashing. This patch does
that and in the process converts ValidateAndSerializeMetadata to use
IPartitionOpener.

Bug: 116802789
Test: manual test
      liblp_test gtest

Change-Id: I3f24cfc6f5607309dc3cded0423326c5ba293d26