Add LOCAL_SDK_VERSION := system_$(VER)

The system_$(VER) can be set in LOCAL_SDK_VERSION, and the apk will use
android_system.jar at build time.
If LOCAL_SDK_VERSION is not defined and this module is installed in
vendor.img, LOCAL_SDK_VERSION is set to system_current.

Bug: 67724799
Test: 1. build && run on taimen
2. LOCAL_SDK_VERSION:=system_27 in ims.apk && build ims.apk && check
the vsdk_v27_intermediates.

Change-Id: I5b11c78b8fcd4a2f2a5e3b141527cd34dbe80018
diff --git a/core/apidiff.mk b/core/apidiff.mk
index 1a7467d..42a1d34 100644
--- a/core/apidiff.mk
+++ b/core/apidiff.mk
@@ -57,8 +57,13 @@
     LOCAL_JAVA_LIBRARIES := android_test_stubs_current $(LOCAL_JAVA_LIBRARIES)
     $(full_target): PRIVATE_BOOTCLASSPATH := $(call java-lib-files, android_test_stubs_current)
   else
-    LOCAL_JAVA_LIBRARIES := sdk_v$(LOCAL_SDK_VERSION) $(LOCAL_JAVA_LIBRARIES)
-    $(full_target): PRIVATE_BOOTCLASSPATH := $(call java-lib-files, sdk_v$(LOCAL_SDK_VERSION))
+    ifneq (,$(call has-system-sdk-version,$(LOCAL_SDK_VERSION)))
+      LOCAL_JAVA_LIBRARIES := system_sdk_v$(call get-numeric-sdk-version,$(LOCAL_SDK_VERSION)) $(LOCAL_JAVA_LIBRARIES)
+      $(full_target): PRIVATE_BOOTCLASSPATH := $(call java-lib-files, system_sdk_v$(call get-numeric-sdk-version,$(LOCAL_SDK_VERSION)))
+    else
+      LOCAL_JAVA_LIBRARIES := sdk_v$(LOCAL_SDK_VERSION) $(LOCAL_JAVA_LIBRARIES)
+      $(full_target): PRIVATE_BOOTCLASSPATH := $(call java-lib-files, sdk_v$(LOCAL_SDK_VERSION))
+    endif
   endif
 else
   LOCAL_JAVA_LIBRARIES := core-oj core-libart ext framework $(LOCAL_JAVA_LIBRARIES)