Turned AudioDecoderFactory into a RefCounted thing to use with scoped_refptr.

First step of AudioDecoderFactory injection CLs. AudioDecoderFactories will be shared, and shared_ptr is currently off the table, so this CL changes the current uses of AudioDecoderFactory from std::unique_ptr to rtc::scoped_refptr.

BUG=webrtc:5805

Review-Url: https://codereview.webrtc.org/1990803004
Cr-Commit-Position: refs/heads/master@{#12815}

webrtc/modules/audio_coding/neteq/decoder_database.cc

@@ -20,10 +20,10 @@
 namespace webrtc {
 
 DecoderDatabase::DecoderDatabase(
-    std::unique_ptr<AudioDecoderFactory> decoder_factory)
+    const rtc::scoped_refptr<AudioDecoderFactory>& decoder_factory)
     : active_decoder_type_(-1),
       active_cng_decoder_type_(-1),
-      decoder_factory_(std::move(decoder_factory)) {}
+      decoder_factory_(decoder_factory) {}
 
 DecoderDatabase::~DecoderDatabase() = default;