Stop using deprecated constraints-based version of CreateAudioSource.

Also did some slight refactoring of the code that turns constraints
into rtc::Optionals. Used a template method to avoid code duplication,
and used the same pattern for "CopyConstraintsIntoAudioOptions" as was
being used for "CopyConstraintsIntoRtcConfiguration".

BUG=webrtc:6752

Review-Url: https://codereview.webrtc.org/2628523003
Cr-Commit-Position: refs/heads/master@{#16063}

webrtc/api/peerconnectioninterface.h

@@ -776,6 +776,7 @@ class PeerConnectionFactoryInterface : public rtc::RefCountInterface {
   virtual rtc::scoped_refptr<AudioSourceInterface> CreateAudioSource(
       const cricket::AudioOptions& options) = 0;
   // Deprecated - use version above.
+  // Can use CopyConstraintsIntoAudioOptions to bridge the gap.
   virtual rtc::scoped_refptr<AudioSourceInterface> CreateAudioSource(
       const MediaConstraintsInterface* constraints) = 0;