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.

Merged-In: I5b11c78b8fcd4a2f2a5e3b141527cd34dbe80018
Change-Id: I5b11c78b8fcd4a2f2a5e3b141527cd34dbe80018
(cherry picked from commit ef212cbe8d6e628bfc40e2db75522b4f1d794766)
diff --git a/core/base_rules.mk b/core/base_rules.mk
index ee3ed6c..50436d1 100644
--- a/core/base_rules.mk
+++ b/core/base_rules.mk
@@ -86,6 +86,7 @@
 endif
 
 include $(BUILD_SYSTEM)/local_vndk.mk
+include $(BUILD_SYSTEM)/local_vsdk.mk
 
 my_module_tags := $(LOCAL_MODULE_TAGS)
 ifeq ($(my_host_cross),true)