Include sdk versioning proto in tethering apex

Allows SdkExtensions to inspect its version to decide what the extension
sdk version is on-device.

Bug: 188787081
Test: m com.android.tethering and inspect
Ignore-AOSP-first: internal topic, will cherry-pick after submission
Change-Id: If2d28b02be871594fed1fbb40b7aad284b553afb

Tethering/apex/Android.bp

@@ -45,6 +45,7 @@ apex {
         "ServiceConnectivityResources",
         "Tethering",
     ],
+    prebuilts: ["current_sdkinfo"],
     manifest: "manifest.json",
     key: "com.android.tethering.key",