Set sdk_version "current" in test package

java_library com.android.test.libs.product is located in product
partition, but its sdk_version isn't set. And it will be forbidden.

So set sdk_version "current"

Test: mma
Bug: 137908189
Change-Id: I3f5a164703f0e5a1f7d43b9f28cda7fa99033186

tests/libs-permissions/Android.bp

@@ -2,6 +2,7 @@ java_library {
     name: "com.android.test.libs.product",
     installable: true,
     product_specific: true,
+    sdk_version: "current",
     srcs: ["product/java/**/*.java"],
     required: ["com.android.test.libs.product.xml"],
 }