Use prebuilt stub org.apache.http.legacy for unbundled build.

Even though the stub org.apache.http.legacy.jar itself can be
built from source against the SDK, its dependency conscrypt must
be built against libcore-art.
(Actually we could remove the droiddoc module apache-http-stubs-gen's
dependency on conscrypt. It would lead doclava to complain error of
unresolved symbols of conscrypt. Such error isn't fatal because the
stub java files don't reference any conscrypt symbols.)

Also by using prebuilt org.apache.http.legacy, unbundled branches don't
need to pull in external/doclava and its many dependency projects.

Bug: 20221594
Change-Id: I6fb4377d35a70ad544cb8a91695e23625365df60