Remove duplicate routing logic from Settings app

Also, remove unused MediaRouter2Manager references.

This is a non-functional change.

Bug: 192657812
Test: Presubmit
Change-Id: I72f5f40e157cea922ad40a2602fba881572fb708

tests/robotests/src/com/android/settings/media/RemoteMediaSliceTest.java

@@ -100,8 +100,8 @@ public class RemoteMediaSliceTest {
         when(remoteSessionInfo.getVolume()).thenReturn(10);
         when(remoteSessionInfo.isSystemSession()).thenReturn(false);
         mRoutingSessionInfos.add(remoteSessionInfo);
-        when(sMediaDeviceUpdateWorker.getActiveRemoteMediaDevice()).thenReturn(
-                mRoutingSessionInfos);
+        when(sMediaDeviceUpdateWorker.getActiveRemoteMediaDevices())
+                .thenReturn(mRoutingSessionInfos);
     }
 
     @Test