Make libsparse vendor_available

We are porting cuttlefish to Android, it currently uses libsparse to
desparse Android filesystems to be used directly as disk images by
crosvm. Making this library available to 'vendor' enables us to support
desparsing downloaded images on-device as well.

Bug: 167675429
Change-Id: I8ae4a916cfb0b2002dd9e462f32e8cb0e533d821