Apply @hide / @SystemApi to android.telecom.*

Move the android.telecom.* namespace back to @hide, and also mark it
with @SystemApi so that system-privileged apps can use them.

Bug: 18302450
Change-Id: I33ae1b9b0dfdb1c5eff51ca3c829196bcfc9411c

telecomm/java/android/telecom/ConnectionService.java

@@ -17,6 +17,7 @@
 package android.telecom;
 
 import android.annotation.SdkConstant;
+import android.annotation.SystemApi;
 import android.app.Service;
 import android.content.ComponentName;
 import android.content.Intent;
@@ -71,7 +72,9 @@ import java.util.concurrent.ConcurrentHashMap;
  * receives call-commands such as answer, reject, hold and disconnect.
  * <p>
  * When there are no more live calls, telecom will unbind from the {@code ConnectionService}.
+ * @hide
  */
+@SystemApi
 public abstract class ConnectionService extends Service {
     /**
      * The {@link Intent} that must be declared as handled by the service.