fs_mgr: Allow using major:minor device strings in CreateLogicalPartition.

IPartitionOpener is useful for writing gtests, however, it can't easily
be used with CreateLogicalPartition. fs_mgr assumes the super partition
name will map to /dev/block/by-name/super whereas in tests we want to
redirect it to a different block device.

This CL makes two changes. First, it adds a new method to IPartitionOpener
to return a "device string" for a device name. The string must either be
an absolute path (for example /dev/block/by-name/super) or a major:minor
sequence, since device-mapper will accept either.

Second, CreateLogicalPartition now accepts an optional IPartitionOpener.
When converting block devices to paths, it uses the opener instead of
automatically prepending /dev/block/by-name.

Bug: 139204329
Test: liblp_test gtest
      libsnapshot_test gtest

Change-Id: Id6b3120cc2ef5c0dd941b29ff96215ad3c8ec848