Self Managed CS Implementation

- Add API guide for developers giving an overview of what they need
to implement.
- Add @hide setLabel method in PhoneAccount; used when registering a
self-managed CS's phone account; we override it with their app label to
ensure they don't try to give themselves another name.

Test: Unit
Bug: 34159263
Change-Id: I0c890dc2feeb3ee438a80ed425db2aaf1f4c1fdd

telecomm/java/android/telecom/Connection.java

@@ -435,6 +435,18 @@ public abstract class Connection extends Conferenceable {
     public static final String EXTRA_ANSWERING_DROPS_FG_CALL =
             "android.telecom.extra.ANSWERING_DROPS_FG_CALL";
 
+    /**
+     * String connection extra key set on a {@link Connection} in {@link Connection#STATE_RINGING}
+     * state to indicate the name of the third-party app which is responsible for the current
+     * foreground call.
+     * <p>
+     * Used when {@link #EXTRA_ANSWERING_DROPS_FG_CALL} is true to ensure that the default Phone app
+     * is able to inform the user that answering the new incoming call will cause a call owned by
+     * another app to be dropped when the incoming call is answered.
+     */
+    public static final String EXTRA_ANSWERING_DROPS_FG_CALL_APP_NAME =
+            "android.telecom.extra.ANSWERING_DROPS_FG_CALL_APP_NAME";
+
     /**
      * Boolean connection extra key on a {@link Connection} which indicates that adding an
      * additional call is disallowed.