Introduce TARGET_BUILD_UNBUNDLED.

Setting this flag enables unbundled building, i.e. without support for
building the system image and other platform targets. This
functionality was previously enabled by TARGET_BUILD_APPS, and setting
that still implies TARGET_BUILD_UNBUNDLED.

This helps unbundled builds that aren't apps, e.g. ART runtest builds.
Specifically, with the topic of the child CL
https://r.android.com/1324517 TARGET_BUILD_UNBUNDLED does not imply
disabling dexpreopting, unlike TARGET_BUILD_APPS.

TODO: There may still be app-specific conditions that are incorrectly
controlled by TARGET_BUILD_UNBUNDLED, in particular on the Soong side
through config.UnbundledBuild().

Test: Flash & boot
Test: TH, in particular builds green on ub-launcher3-master
Bug: 157549171
Change-Id: Ic09fc879117ee06cab5444edfc280ed2b52d2870
diff --git a/core/soong_config.mk b/core/soong_config.mk
index c3e8c2b..53c7b0d 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -39,7 +39,7 @@
 $(call add_json_str,  Platform_min_supported_target_sdk_version, $(PLATFORM_MIN_SUPPORTED_TARGET_SDK_VERSION))
 
 $(call add_json_bool, Allow_missing_dependencies,        $(ALLOW_MISSING_DEPENDENCIES))
-$(call add_json_bool, Unbundled_build,                   $(TARGET_BUILD_APPS))
+$(call add_json_bool, Unbundled_build,                   $(TARGET_BUILD_UNBUNDLED))
 $(call add_json_bool, Unbundled_build_sdks_from_source,  $(UNBUNDLED_BUILD_SDKS_FROM_SOURCE))
 $(call add_json_bool, Pdk,                               $(filter true,$(TARGET_BUILD_PDK)))