Add tethering bootclasspath fragment to its sdk

This subsumes the java_sdk_library in the sdk.

Bug: 188427719
Test: build_mainline_modules.sh
Change-Id: Ib2ebb3893135b4dd91812f1f1b0915540cfd64cb

Tethering/Android.bp

@@ -158,7 +158,5 @@ android_app {
 
 sdk {
     name: "tethering-module-sdk",
-    java_sdk_libs: [
+    bootclasspath_fragments: ["com.android.tethering-bootclasspath-fragment"],
-        "framework-tethering",
-    ],
 }