RRO shouldn't depend on frameworks or system(_ext) app when TARGET_BUILD_UNBUNDLED

In the case of unbundled build, the module in vendor should use
system(_ext) module by prebuilt one. But RRO depends on system module
directly depending on some conditions(packages exporting resources)

In this change,
 1. Temporarily make LOCAL_RES_LIBRARIES empty(For now, auto generated
 RRO doesn't use overlaid package's resources), enable it when prebuilts
 are ready.
 2. According to (1), its SDK_VERSION can be current)

Bug: 187404676
Test: TARGET_BUILD_UNBUNDLED_IMAGE m vendorimage, and check if there is
no build error regarding RRO.

Change-Id: I94e3122372dd20c942b2c858070a6ca797312792
Merged-In: I94e3122372dd20c942b2c858070a6ca797312792
(cherry picked from commit f93c13d51bff2478d4bcf48c808bc78a5ce0b989)

core/generate_enforce_rro.mk

@@ -29,9 +29,12 @@ $(rro_android_manifest_file): build/make/tools/generate-enforce-rro-android-mani
 
 LOCAL_PATH:= $(intermediates)
 
+# TODO(b/187404676): remove this condition when the prebuilt for packges exporting resource exists.
+ifeq (,$(TARGET_BUILD_UNBUNDLED))
 ifeq ($(enforce_rro_use_res_lib),true)
   LOCAL_RES_LIBRARIES := $(enforce_rro_source_module)
 endif
+endif
 
 LOCAL_FULL_MANIFEST_FILE := $(rro_android_manifest_file)
 
@@ -45,8 +48,9 @@ else ifeq (vendor,$(enforce_rro_partition))
 else
   $(error Unsupported partition. Want: [vendor/product] Got: [$(enforce_rro_partition)])
 endif
-
-ifneq (,$(LOCAL_RES_LIBRARIES))
+ifneq (,$(TARGET_BUILD_UNBUNDLED))
+  LOCAL_SDK_VERSION := current
+else ifneq (,$(LOCAL_RES_LIBRARIES))
   # Technically we are linking against the app (if only to grab its resources),
   # and because it's potentially not building against the SDK, we can't either.
   LOCAL_PRIVATE_PLATFORM_APIS := true