Fix link-type check warnings on kotlin-stdlib

This change fixes the link-type check warning from an app built with SDK
to the prebuilt module kotlin-stdlib. The warnings were triggered since
the prebuilt module is declared without the sdk_version property and thus
it is incorrectly identified as a lib using private APIs.

Fixing the warning simply by setting sdk_version property to
"core_current". The value was chosen instead of "current" because the
kotlin library does not have any dependency to Android-specific APIs.

Bug: 69899800
Test: m -j android-arch-room-migration is successful and does not show
any link-type warning (for now the test must be run in the internal
master as dependency to kotlin-stdlib is not yet on AOSP)

Change-Id: I66c296bea3a92540a8a0546fb824a8aa2118857e
diff --git a/Android.bp b/Android.bp
index 8822d1e..3c46db4 100644
--- a/Android.bp
+++ b/Android.bp
@@ -2,4 +2,5 @@
     name: "kotlin-stdlib",
     host_supported: true,
     jars: ["lib/kotlin-stdlib.jar"],
+    sdk_version: "core_current",
 }