liblp: Always align the first usable sector.

Align the first usable sector to the logical block size, if no other
alignment was specified. This fixes a bunch of warnings during certain
gtests (ones with unaligned metadata sizes). The warnings were coming
from MetadataBuilder::GrowPartition() which expects the first sector
to always be block-aligned.

Bug: 116802789
Test: liblp_test gtest
Change-Id: I8dcf502aa4c2ba0674c5b4dcb77a274f300ff0a3