Fix genrule tool dependencies when a prebuilt tool is preferred.

Since the genrule tool dep mutator runs after the prebuilt mutators,
this adds a helper function android.PrebuiltGetPreferred to resolve the
source or prebuilt as appropriate.

Test: m SOONG_CONFIG_art_module_source_build=false droid
  in internal
Bug: 214292395
Change-Id: I1a208fd048b998f9f19ad1f45d8389decda2cb9e