Fix NPE happened in the constructor of SystemMediaRoute2Provider

Bug: 149751047
Test: build
Change-Id: I7ce7bb3aa6f82aec8ca3cbd0b04cd68c3f88ea1f

services/core/java/com/android/server/media/SystemMediaRoute2Provider.java

@@ -214,6 +214,8 @@ class SystemMediaRoute2Provider extends MediaRoute2Provider {
      * Updates the mSessionInfo. Returns true if the session info is changed.
      */
     boolean updateSessionInfosIfNeededLocked() {
+        // Prevent to execute this method before mBtRouteProvider is created.
+        if (mBtRouteProvider == null) return false;
         RoutingSessionInfo oldSessionInfo = mSessionInfos.isEmpty() ? null : mSessionInfos.get(0);
 
         RoutingSessionInfo.Builder builder = new RoutingSessionInfo.Builder(