liblp: MetadataBuilder::GetBlockDevicePartitionName

Allow to retrieve name of a block device partition at
a certain index. If the index is out of range, return
empty string.

This is needed for an hypothetical edge case when an
intersection of two LinearExtent needs to be computed,
and the two metadata have a different list of block
device names. In reality, Virtual A/B devices are always
launched with dynamic partitions, so it will always return
"super" for index 0 and "" otherwise.

Test: liblp_test_static
Bug: 135752105

Change-Id: I9ea59edefdc41d0e69e3644aa2452676372938b4