Add @SystemApi to missing TelecomManager methods

A few methods (which should be exposed) were missing @SystemApi
annotations.

TelecomManager#addNewIncomingCall
TelecomManager#registerPhoneAccount
TelecomManager#unregisterPhoneAccount

Bug: 17786491
Change-Id: Idf318cf6238668762c00ed0913a9c0ccd5bcb884

telecomm/java/android/telecom/TelecomManager.java

@@ -584,6 +584,7 @@ public class TelecomManager {
      * @param account The complete {@link PhoneAccount}.
      * @hide
      */
+    @SystemApi
     public void registerPhoneAccount(PhoneAccount account) {
         try {
             if (isServiceConnected()) {
@@ -600,6 +601,7 @@ public class TelecomManager {
      * @param accountHandle A {@link PhoneAccountHandle} for the {@link PhoneAccount} to unregister.
      * @hide
      */
+    @SystemApi
     public void unregisterPhoneAccount(PhoneAccountHandle accountHandle) {
         try {
             if (isServiceConnected()) {
@@ -800,6 +802,7 @@ public class TelecomManager {
      *            {@link ConnectionService#onCreateIncomingConnection}.
      * @hide
      */
+    @SystemApi
     public void addNewIncomingCall(PhoneAccountHandle phoneAccount, Bundle extras) {
         try {
             if (isServiceConnected()) {