Optional: Use nullopt and implicit construction in /modules/audio_device

Changes places where we explicitly construct an Optional to instead use
nullopt or the requisite value type only.

This CL was uploaded by git cl split.

Bug: None
Change-Id: I9f581b44a43db701300e100a3ff70d90e3dd4f6e
Reviewed-on: https://webrtc-review.googlesource.com/23572
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Oskar Sundbom <ossu@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21581}

modules/audio_device/audio_device_unittest.cc

@@ -204,7 +204,7 @@ class LatencyAudioStream : public AudioStream {
       {
         rtc::CritScope lock(&lock_);
         if (!pulse_time_) {
-          pulse_time_ = rtc::Optional<int64_t>(rtc::TimeMillis());
+          pulse_time_ = rtc::TimeMillis();
         }
       }
       constexpr int16_t impulse = std::numeric_limits<int16_t>::max();