Moved codec-specific audio packet splitting into decoders.

There's still some code run specifically for Opus w/ FEC. It will be
addressed in a separate CL.

BUG=webrtc:5805

Review-Url: https://codereview.webrtc.org/2326003002
Cr-Commit-Position: refs/heads/master@{#14319}

webrtc/modules/audio_coding/neteq/neteq_impl_unittest.cc

@@ -336,9 +336,6 @@ TEST_F(NetEqImplTest, InsertPacket) {
   EXPECT_CALL(*mock_payload_splitter_, SplitFec(_, _))
       .Times(2)
       .WillRepeatedly(Return(PayloadSplitter::kOK));
-  EXPECT_CALL(*mock_payload_splitter_, SplitAudio(_, _))
-      .Times(2)
-      .WillRepeatedly(Return(PayloadSplitter::kOK));
 
   // Insert first packet.
   neteq_->InsertPacket(rtp_header, payload, kFirstReceiveTime);