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/main/acm2/audio_coding_module_unittest_oldapi.cc

@@ -988,9 +988,6 @@ TEST_F(AcmReceiverBitExactnessOldApi, MAYBE_48kHzOutputExternalDecoder) {
   MockAudioDecoder mock_decoder;
   // Set expectations on the mock decoder and also delegate the calls to the
   // real decoder.
-  EXPECT_CALL(mock_decoder, Init())
-      .Times(AtLeast(1))
-      .WillRepeatedly(Invoke(&decoder, &AudioDecoderPcmU::Init));
   EXPECT_CALL(mock_decoder, IncomingPacket(_, _, _, _, _))
       .Times(AtLeast(1))
       .WillRepeatedly(Invoke(&decoder, &AudioDecoderPcmU::IncomingPacket));