liblp: Allow "flashing" files on the host.

UpdatePartitionTable expects to be run on a device, which is a
reasonable assumption. However to implement lpadd (offline modification
of super images), it's useful to be able to run these methods, and
IPartitionOpener allows us to masquerade a flie as a block device.

The only catch is that we can't query partition information on the host.
Since this logic is a protection mechanism, just surround it with
an ifdef for __ANDROID__.

Bug: 144468779
Test: lpadd
Change-Id: I3c50e5677fa1dd59f2d4216038d3d4fbfb6edcc0