Add call waiting to Ringer.java.

When an incoming call is in the background, play call waiting instead of
ringer.

Bug: 13674415

Change-Id: I9d450202774c6890794fa318fdab01290b4a7eb2
diff --git a/src/com/android/telecomm/Call.java b/src/com/android/telecomm/Call.java
index 6aae5ca..2db5b4e 100644
--- a/src/com/android/telecomm/Call.java
+++ b/src/com/android/telecomm/Call.java
@@ -461,6 +461,17 @@
         }
     }
 
+    boolean isActive() {
+        switch (mState) {
+            case ACTIVE:
+            case POST_DIAL:
+            case POST_DIAL_WAIT:
+                return true;
+            default:
+                return false;
+        }
+    }
+
     Bundle getExtras() {
         return mExtras;
     }