Expose a meta-data value to allow dialer ringing

This CL exposes a new meta-data, IN_CALL_SERVICE_RINGING. If this is set
to true then ringing is played by the dialer instead of Telecom.

This CL also adds a new silenceRinger() API to InCallService. This is
needed to implement ringer silence on volume key down.

BUG: 22857261
Change-Id: I498538282eddbb727104f5b879f25adbef4e6cf6
diff --git a/telecomm/java/android/telecom/TelecomManager.java b/telecomm/java/android/telecom/TelecomManager.java
index 497864e..72ff272 100644
--- a/telecomm/java/android/telecom/TelecomManager.java
+++ b/telecomm/java/android/telecom/TelecomManager.java
@@ -308,6 +308,15 @@
             "android.telecom.IN_CALL_SERVICE_CAR_MODE_UI";
 
     /**
+     * A boolean meta-data value indicating whether an {@link InCallService} implements ringing.
+     * Dialer implementations (see {@link #getDefaultDialerPackage()}) which would also like to
+     * override the system provided ringing should set this meta-data to {@code true} in the
+     * manifest registration of their {@link InCallService}.
+     */
+    public static final String METADATA_IN_CALL_SERVICE_RINGING =
+            "android.telecom.IN_CALL_SERVICE_RINGING";
+
+    /**
      * The dual tone multi-frequency signaling character sent to indicate the dialing system should
      * pause for a predefined period.
      */