Add an Android.bp for Soong

So that we can build xz as part of the AOSP build-tools branch, and
deliver prebuilts of the host tool `xz` into the android tree for use
during the build. (We're currently using whatever xz is in the PATH)

Bug: 36130900
Test: build xz for darwin and linux
Test: do a build with the built xz
Change-Id: I6f953f821f55ef785ada2478de183c718693009e