Fix Launcher3Tests SDK version

In a879f9c481bb324a7ec38a238c8a7f306b58b868, the ub-launcher-aosp-tapl
was switched from current to system_current. Since Launcher3Tests links
against this library, we require it to use the system stubs as well.

But I broke the check in the build system while doing some cleanup, so
we never noticed. I'm fixing the check in:
https://android-review.googlesource.com/c/platform/build/+/1156840

Bug: 143748700
Test: treehugger
Change-Id: Ifcf47b19f7ed8f907e437325b4fd6a599fcca4cf
diff --git a/tests/Android.mk b/tests/Android.mk
index 31a9960..694f060 100644
--- a/tests/Android.mk
+++ b/tests/Android.mk
@@ -56,7 +56,7 @@
     LOCAL_PRIVATE_PLATFORM_APIS := true
     LOCAL_STATIC_JAVA_LIBRARIES += launcher-aosp-tapl
 else
-    LOCAL_SDK_VERSION := 28
+    LOCAL_SDK_VERSION := system_28
     LOCAL_MIN_SDK_VERSION := 21
     LOCAL_STATIC_JAVA_LIBRARIES += ub-launcher-aosp-tapl
 endif