liblp: Allow the super partition to span multiple block devices.

This patch allows the block device table in LpMetadataHeader to contain
additional partitions. MetadataBuilder can now resize partitions such
that are allocated across block devices as needed, however, it attempts
to minimize this by grouping free regions by device.

Bug: 116802789
Test: liblp_test gtests
      device with super partition flashes and boots

Change-Id: I9cf74c8925faf154703eeba2a26546a152efcaa2