Revert "Remove rtc::Optional alias and api:optional target"

This reverts commit 6f5b0f920af08d66e6b77ee4f91ade5797145368.

Reason for revert: Breaks internal project.

Original change's description:
> Remove rtc::Optional alias and api:optional target
> 
> Update left-overs where old target still was used.
> 
> Bug: webrtc:9078
> Change-Id: I2162c928091fc4ff1dea33a3f03adbe47207d206
> Reviewed-on: https://webrtc-review.googlesource.com/84740
> Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#23913}

TBR=danilchap@webrtc.org,kwiberg@webrtc.org

Change-Id: I95f5ec33520b823c3d0c9cb83d945d6a15355367
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:9078
Reviewed-on: https://webrtc-review.googlesource.com/88140
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23921}
diff --git a/modules/audio_coding/neteq/tools/neteq_input.h b/modules/audio_coding/neteq/tools/neteq_input.h
index 1691585..a13a86e 100644
--- a/modules/audio_coding/neteq/tools/neteq_input.h
+++ b/modules/audio_coding/neteq/tools/neteq_input.h
@@ -84,18 +84,18 @@
 class TimeLimitedNetEqInput : public NetEqInput {
  public:
   TimeLimitedNetEqInput(std::unique_ptr<NetEqInput> input, int64_t duration_ms);
-  absl::optional<int64_t> NextPacketTime() const override;
-  absl::optional<int64_t> NextOutputEventTime() const override;
+  rtc::Optional<int64_t> NextPacketTime() const override;
+  rtc::Optional<int64_t> NextOutputEventTime() const override;
   std::unique_ptr<PacketData> PopPacket() override;
   void AdvanceOutputEvent() override;
   bool ended() const override;
-  absl::optional<RTPHeader> NextHeader() const override;
+  rtc::Optional<RTPHeader> NextHeader() const override;
 
  private:
   void MaybeSetEnded();
 
   std::unique_ptr<NetEqInput> input_;
-  const absl::optional<int64_t> start_time_ms_;
+  const rtc::Optional<int64_t> start_time_ms_;
   const int64_t duration_ms_;
   bool ended_ = false;
 };