Set sdk_version 'system_current'

This android_library used too broader sdk_version,
even though there is no hidden API usage.

Bug: 143736861
Test: mma
Change-Id: Ia888a230c2bf41e6921061b14eeb9c18fd4661e7
diff --git a/Android.bp b/Android.bp
index 764dc49..3783408 100644
--- a/Android.bp
+++ b/Android.bp
@@ -83,6 +83,7 @@
 
     min_sdk_version: "28",
     target_sdk_version: "28",
+    sdk_version: "system_current",
 }
 
 android_app {