Use scratch memory instead of perm buffer for the RFFT kernel's work area. (#2542) BUG=333876877

diff --git a/signal/micro/kernels/rfft.cc b/signal/micro/kernels/rfft.cc
index 2ee8e7a..c9472b0 100644
--- a/signal/micro/kernels/rfft.cc
+++ b/signal/micro/kernels/rfft.cc

@@ -48,6 +48,7 @@
   int32_t output_length;
   TfLiteType fft_type;
   T* work_area;
+  int scratch_buffer_index;
   int8_t* state;
 };
 
@@ -65,9 +66,6 @@
   params->fft_length = fbw.ElementAsInt32(kFftLengthIndex);
   params->fft_type = typeToTfLiteType<T>();
 
-  params->work_area = static_cast<T*>(context->AllocatePersistentBuffer(
-      context, params->fft_length * sizeof(T)));
-
   size_t state_size = (*get_needed_memory_func)(params->fft_length);
   params->state = static_cast<int8_t*>(
       context->AllocatePersistentBuffer(context, state_size * sizeof(int8_t)));
@@ -103,6 +101,8 @@
   params->output_length =
       output_shape.Dims(output_shape.DimensionsCount() - 1) / 2;
 
+  context->RequestScratchBufferInArena(context, params->fft_length * sizeof(T),
+                                       &params->scratch_buffer_index);
   micro_context->DeallocateTempTfLiteTensor(input);
   micro_context->DeallocateTempTfLiteTensor(output);
   return kTfLiteOk;
@@ -122,15 +122,17 @@
       tflite::micro::GetEvalOutput(context, node, kOutputTensor);
   Complex<T>* output_data = tflite::micro::GetTensorData<Complex<T>>(output);
 
+  T* work_area = static_cast<T*>(
+      context->GetScratchBuffer(context, params->scratch_buffer_index));
+
   for (int input_idx = 0, output_idx = 0; input_idx < params->input_size;
        input_idx += params->input_length, output_idx += params->output_length) {
-    memcpy(params->work_area, &input_data[input_idx],
-           sizeof(T) * params->input_length);
+    memcpy(work_area, &input_data[input_idx], sizeof(T) * params->input_length);
     // Zero pad input to FFT length
-    memset(&params->work_area[params->input_length], 0,
+    memset(&work_area[params->input_length], 0,
            sizeof(T) * (params->fft_length - params->input_length));
 
-    (*apply_func)(params->state, params->work_area, &output_data[output_idx]);
+    (*apply_func)(params->state, work_area, &output_data[output_idx]);
   }
   return kTfLiteOk;
 }