Update Android Model Benchmark Tool to support Flex delegate

Also updated README.md

PiperOrigin-RevId: 324157303
Change-Id: I40bd6fbac0c5aafa12a3867ab2161879df494fee

tensorflow/lite/tools/benchmark/android/BUILD

@@ -37,6 +37,7 @@ tflite_jni_binary(
         "jni/**/*.h",
     ]),
     deps = [
+        "//tensorflow/lite/delegates/flex:delegate",
         "//tensorflow/lite/java/jni",
         "//tensorflow/lite/tools/benchmark:benchmark_tflite_model_lib",
     ],

tensorflow/lite/tools/benchmark/android/README.md

@@ -27,9 +27,12 @@ to edit the `WORKSPACE` to configure the android NDK/SDK.
 
 ```
 bazel build -c opt \
+  --config=monolithic \
   --config=android_arm64 \
+  --cxxopt='--std=c++14' \
   tensorflow/lite/tools/benchmark/android:benchmark_model
 ```
+Note: "--config=monolithic" was added to support TF ops via [Flex delegate](https://www.tensorflow.org/lite/guide/ops_select).
 
 (Optional) To enable Hexagon delegate with `--use_hexagon=true` option, you can
 download and install the libraries as the guided in [hexagon delegate]