Force building ART from source for AOSP products.

This is a workaround for the problem that we cannot control from make
which APEX gets picked for deapexing.

Cherry-picked from http://ag/15107826.

Test: env TARGET_PRODUCT=aosp_cf_x86_64_phone \
          SOONG_CONFIG_art_module_source_build=false \
        build/soong/soong_ui.bash \
        --dumpvar-mode SOONG_CONFIG_art_module_source_build
  This shows true
Test: env TARGET_PRODUCT=cf_x86_64_phone \
          SOONG_CONFIG_art_module_source_build=false \
        build/soong/soong_ui.bash \
        --dumpvar-mode SOONG_CONFIG_art_module_source_build
  This shows false in internal
Bug: 192006406
Bug: 172480615
Change-Id: I8990b726affeb08eabf130d20be45129a7c1c58d
Merged-In: I8990b726affeb08eabf130d20be45129a7c1c58d