Replace rtc::Optional with absl::optional in modules/audio processing

This is a no-op change because rtc::Optional is an alias to absl::optional

This CL generated by running script with parameter 'modules/audio_processing'

find $@ -type f \( -name \*.h -o -name \*.cc \) \
-exec sed -i 's|rtc::Optional|absl::optional|g' {} \+ \
-exec sed -i 's|rtc::nullopt|absl::nullopt|g' {} \+ \
-exec sed -i 's|#include "api/optional.h"|#include "absl/types/optional.h"|' {} \+

find $@ -type f -name BUILD.gn \
-exec sed -r -i 's|"(../)*api:optional"|"//third_party/abseil-cpp/absl/types:optional"|' {} \+;

git cl format

Bug: webrtc:9078
Change-Id: Id29f8de59dba704787c2c38a3d05c60827c181b0
Reviewed-on: https://webrtc-review.googlesource.com/83982
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23653}

modules/audio_processing/echo_detector/circular_buffer.h

@@ -13,7 +13,7 @@
 
 #include <vector>
 
-#include "api/optional.h"
+#include "absl/types/optional.h"
 
 namespace webrtc {
 
@@ -24,7 +24,7 @@ struct CircularBuffer {
   ~CircularBuffer();
 
   void Push(float value);
-  rtc::Optional<float> Pop();
+  absl::optional<float> Pop();
   size_t Size() const { return nr_elements_in_buffer_; }
   // This function fills the buffer with zeros, but does not change its size.
   void Clear();