internal/xds: change xds_resolver to use dependency manager (#8711)

This change is part of
[A74](https://github.com/grpc/proposal/blob/master/A74-xds-config-tears.md)
implementation.

This PR removes the listener and route watchers from resolver and
changes it so that we get the resources from xds dependency manager.

RELEASE NOTES: 
* xds/resolver: 
* Changes the behavior such that getting no matching virtual host in a
route resource will now drop any previous resource and report the error.
* Changes the behavior so that receiving a configuration error (LDS/RDS
ambient error) after a successful update will now only be logged, and
the system will continue using the previous resource to avoid transient
channel failures

Author: eshitachandwani

internal/xds/resolver/helpers_test.go

@@ -191,26 +191,6 @@ func verifyNoUpdateFromResolver(ctx context.Context, t *testing.T, stateCh chan
 	}
 }
 
-// waitForErrorFromResolver waits for the resolver to push an error and verifies
-// that it matches the expected error and contains the expected node ID.
-func waitForErrorFromResolver(ctx context.Context, errCh chan error, wantErr, wantNodeID string) error {
-	select {
-	case <-ctx.Done():
-		return fmt.Errorf("timeout when waiting for error to be propagated to the ClientConn")
-	case gotErr := <-errCh:
-		if gotErr == nil {
-			return fmt.Errorf("got nil error from resolver, want %q", wantErr)
-		}
-		if !strings.Contains(gotErr.Error(), wantErr) {
-			return fmt.Errorf("got error from resolver %q, want %q", gotErr, wantErr)
-		}
-		if !strings.Contains(gotErr.Error(), wantNodeID) {
-			return fmt.Errorf("got error from resolver %q, want nodeID %q", gotErr, wantNodeID)
-		}
-	}
-	return nil
-}
-
 func verifyResolverError(gotErr error, wantCode codes.Code, wantErr, wantNodeID string) error {
 	if gotErr == nil {
 		return fmt.Errorf("got nil error from resolver, want error with code %v", wantCode)