AudioDecoder: Replace Init() with Reset()

The Init() method was previously used to initialize and reset
decoders, and returned an error code. The new Reset() method is used
for reset only; the constructor is now responsible for fully
initializing the AudioDecoder.

Reset() doesn't return an error code; it turned out that none of the
functions it ended up calling could actually fail, so this CL removes
their error return codes as well.

R=henrik.lundin@webrtc.org

Review URL: https://codereview.webrtc.org/1319683002 .

Cr-Commit-Position: refs/heads/master@{#9798}

webrtc/modules/audio_coding/codecs/opus/opus_unittest.cc

@@ -376,7 +376,7 @@ TEST_P(OpusTest, OpusDecodeInit) {
                 kOpus20msFrameSamples, opus_decoder_, output_data_decode,
                 &audio_type)));
 
-  EXPECT_EQ(0, WebRtcOpus_DecoderInit(opus_decoder_));
+  WebRtcOpus_DecoderInit(opus_decoder_);
 
   EXPECT_EQ(kOpus20msFrameSamples,
             static_cast<size_t>(WebRtcOpus_Decode(