Make RouteSessionInfo#getId() return String rather than primitive int

This CL makes the session have String ID rather than
primitive integer ID.

Also, this CL makes RouteSessionInfo#getId() return a unique ID
when it is called for routes which were get via
MediaRouter2 / MediaRouter2Manager.

Bug: 147394941
Test: atest mediaroutertest
Change-Id: I715617243f1561cb00eb1cca6346ba772813516a
diff --git a/services/core/java/com/android/server/media/MediaRoute2Provider.java b/services/core/java/com/android/server/media/MediaRoute2Provider.java
index 9ca302e..9c9a412 100644
--- a/services/core/java/com/android/server/media/MediaRoute2Provider.java
+++ b/services/core/java/com/android/server/media/MediaRoute2Provider.java
@@ -47,11 +47,11 @@
 
     public abstract void requestCreateSession(String packageName, String routeId,
             String routeType, long requestId);
-    public abstract void releaseSession(int sessionId);
+    public abstract void releaseSession(String sessionId);
 
-    public abstract void selectRoute(int sessionId, String routeId);
-    public abstract void deselectRoute(int sessionId, String routeId);
-    public abstract void transferToRoute(int sessionId, String routeId);
+    public abstract void selectRoute(String sessionId, String routeId);
+    public abstract void deselectRoute(String sessionId, String routeId);
+    public abstract void transferToRoute(String sessionId, String routeId);
 
     public abstract void sendControlRequest(String routeId, Intent request);
     public abstract void requestSetVolume(String routeId, int volume);