Restrict visibility into the sdkext package

Only the stubs should be visible to the outside world. However,
the sources also need to be exposed for now.

Bug: 137191822
Test: m
Change-Id: Id051cc928c2c5050a6df6470e9ee221f178fd835
diff --git a/apex/sdkext/Android.bp b/apex/sdkext/Android.bp
index 6b52481..b9071f8 100644
--- a/apex/sdkext/Android.bp
+++ b/apex/sdkext/Android.bp
@@ -12,6 +12,10 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+package {
+    default_visibility: [":__subpackages__"],
+}
+
 apex {
     name: "com.android.sdkext",
     manifest: "manifest.json",