Always use ART sources for Fuchsia.

Fuchsia currently adds a number of ART internal packages in
device/google/fuchsia/fuchsia.mk, so it cannot use prebuilts.

Test: m TARGET_PRODUCT=fuchsia_arm64 \
        TARGET_BUILD_VARIANT={user,eng} nothing
  with SOONG_CONFIG_art_module_source_build set to false by default.
Bug: 172480615
Change-Id: I6af54accd8cc724b92072ae112960ace8faa70d5