Fix ext_proc mock response handling in ext_proc client interceptor test (grpc#12975)

In
`ExternalProcessorClientInterceptorTest.clientInterceptor_contextPropagatedToStartCall()`,
the mock ext_proc service blindly returned a `ProcessingResponse` with
`setRequestHeaders` for every incoming `ProcessingRequest`. When the
downstream RPC finished and sent back response headers, the interceptor
sent a `ProcessingRequest` with `responseHeaders`. The mock's blind
`requestHeaders` reply caused the interceptor to detect an out-of-order
protocol error, triggering `internalOnError()` which called
`delayedCall.cancel()` on the executor thread.

This async cancellation on the executor thread raced with the test
code's cleanup `proxyCall.cancel()` on the test runner thread, causing
TSAN to report a data race on the non-volatile field
`ClientCallImpl.cancelCalled`.

This commit fixes the test mock to check the request type
(`hasRequestHeaders()` vs. `hasResponseHeaders()`) before responding,
matching the behavior in
`clientInterceptor_contextPropagatedToListenerCallbacks()`.

Note: The unsynchronized cancellation between the application thread and
an interceptor's cancellation on an executor thread, while technically a
data race from TSAN's perspective due to `ClientCallImpl.cancelCalled`
not being volatile, is not consequential in practice because the
underlying `stream.cancel()` and `CancellationHandler.tearDown()` are
thread-safe and idempotent. Therefore, modifying `cancelCalled` in
`ClientCallImpl` is not necessary.

Internal TSAN test failure [link](http://shortn/_sh1PGgNkbQ)

Author: kannanjgithub

xds/src/test/java/io/grpc/xds/ExternalProcessorClientInterceptorTest.java

@@ -14309,9 +14309,15 @@ public StreamObserver<ProcessingRequest> process(
             return new StreamObserver<ProcessingRequest>() {
               @Override
               public void onNext(ProcessingRequest request) {
-                responseObserver.onNext(ProcessingResponse.newBuilder()
-                    .setRequestHeaders(HeadersResponse.newBuilder().build())
-                    .build());
+                if (request.hasRequestHeaders()) {
+                  responseObserver.onNext(ProcessingResponse.newBuilder()
+                      .setRequestHeaders(HeadersResponse.newBuilder().build())
+                      .build());
+                } else if (request.hasResponseHeaders()) {
+                  responseObserver.onNext(ProcessingResponse.newBuilder()
+                      .setResponseHeaders(HeadersResponse.newBuilder().build())
+                      .build());
+                }
               }
 
               @Override