Add handover permission, fill in some missing API gaps.

Adding the ACCEPT_HANDOVER runtime permission which an app must have in
order to accept handovers (this is per design).

Adding missing onHandoverComplete method in the android.telecom.Connection
API (per design).

Finishing plumbing for android.telecom.Call#onHandoverComplete API.

Fix issue where the new handover API methods would never get called; the
legacy handover extra was being used in this case when it should not have
been.

Bug: 65415068
Test: Verified using new CTS tests
Change-Id: If1558f6a23911862c02ac5b18fb62d86911ed7e2

telecomm/java/android/telecom/TelecomManager.java

@@ -384,6 +384,17 @@ public class TelecomManager {
      */
     public static final String EXTRA_IS_HANDOVER = "android.telecom.extra.IS_HANDOVER";
 
+    /**
+     * When {@code true} indicates that a request to create a new connection is for the purpose of
+     * a handover.  Note: This is used with the
+     * {@link android.telecom.Call#handoverTo(PhoneAccountHandle, int, Bundle)} API as part of the
+     * internal communication mechanism with the {@link android.telecom.ConnectionService}.  It is
+     * not the same as the legacy {@link #EXTRA_IS_HANDOVER} extra.
+     * @hide
+     */
+    public static final String EXTRA_IS_HANDOVER_CONNECTION =
+            "android.telecom.extra.IS_HANDOVER_CONNECTION";
+
     /**
      * Parcelable extra used with {@link #EXTRA_IS_HANDOVER} to indicate the source
      * {@link PhoneAccountHandle} when initiating a handover which {@link ConnectionService}