Tethering: build tethering as unbundled APP

Bug: 143195885
Test: -build, flash, boot
      -atest TetheringTests

Change-Id: I09117228c193137f6359fdcfced38352b8a38f57

Tethering/Android.bp

@@ -16,7 +16,7 @@
 
 java_defaults {
     name: "TetheringAndroidLibraryDefaults",
-    platform_apis: true,
+    sdk_version: "system_current",
     srcs: [
         "src/**/*.java",
         ":framework-tethering-shared-srcs",
@@ -81,7 +81,7 @@ cc_library {
 // Common defaults for compiling the actual APK.
 java_defaults {
     name: "TetheringAppDefaults",
-    platform_apis: true,
+    sdk_version: "system_current",
     privileged: true,
     // Build system doesn't track transitive dependeicies for jni_libs, list all the dependencies
     // explicitly.