Stop building host native test in unbundled build

It turned out that building native code for host environment
is not supported in NDK build.  Hence this CL makes the host
native test available only as a part of platform build to
avoid accidental build breakage in unbundled build.

BUG: 18095678
Change-Id: If608da166d5a478358e6890b8db526b4c2c0ab41