liblp: Add an abstraction layer for opening partitions.

This change introduces an IPartitionOpener abstraction. Methods that
interact with live metadata, such as ReadMetadata, UpdatePartitionTable,
and FlashPartitionTable, now require an IPartitionOpener object. Its
purpose is dependency injection: it will make these methods much easier
to test when the super partition spans multiple block devices.

All non-test consumers should be using PartitionOpener, and as such,
some helper methods have been added that automatically create one.

Bug: 116802789
Test: liblp_test gtest
      device with super partition boots
Change-Id: I76725a5830ef643c5007c152c00ccaad8085151f