Introduce TARGET_BUILD_UNBUNDLED_IMAGE

- TARGET_BUILD_UNBUNDLED_IMAGE is similar to TARGET_BUILD_APPS, but
its targets are the unbundled partitions instead of apps.
- Rename TARGET_BUILD_APPS_USE_PREBUILT_SDK to TARGET_BUILD_USE_PREBUILT_SDKS
because it is used even without TARGET_BUILD_APPS.
-Instead of TARGET_BUILD_APPS, use TARGET_BUILD_USE_PREBUILT_SDKS
to build java modules with prebuilt sdks, and propagate to Soong.

Bug: 160390776
Test: TARGET_BUILD_UNBUNDLED_IMAGE=true m vendorimage
Change-Id: Ie096212ccbcca0018baae55e106af693b002c9e5
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 98ab07d..d923c56 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -41,7 +41,7 @@
 $(call add_json_bool, Allow_missing_dependencies,        $(ALLOW_MISSING_DEPENDENCIES))
 $(call add_json_bool, Unbundled_build,                   $(TARGET_BUILD_UNBUNDLED))
 $(call add_json_bool, Unbundled_build_apps,              $(TARGET_BUILD_APPS))
-$(call add_json_bool, Unbundled_build_sdks_from_source,  $(UNBUNDLED_BUILD_SDKS_FROM_SOURCE))
+$(call add_json_bool, Always_use_prebuilt_sdks,          $(TARGET_BUILD_USE_PREBUILT_SDKS))
 $(call add_json_bool, Pdk,                               $(filter true,$(TARGET_BUILD_PDK)))
 
 $(call add_json_bool, Debuggable,                        $(filter userdebug eng,$(TARGET_BUILD_VARIANT)))