Revert "@hide ConnectionService APIs."

This reverts commit 86935c406e4a8bb0e2ad871b51009f3603df9e87.

telecomm/java/android/telecom/ConnectionService.java

@@ -16,7 +16,6 @@
 
 package android.telecom;
 
-import android.annotation.SystemApi;
 import android.annotation.SdkConstant;
 import android.app.Service;
 import android.content.ComponentName;
@@ -43,9 +42,7 @@ import java.util.concurrent.ConcurrentHashMap;
 /**
  * A {@link android.app.Service} that provides telephone connections to processes running on an
  * Android device.
- * @hide
  */
-@SystemApi
 public abstract class ConnectionService extends Service {
     /**
      * The {@link Intent} that must be declared as handled by the service.