Make an enum class out of NetEqDecoder, and hide the neteq_decoders_ table

This operation was relatively simple, since no one was doing anything
fishy with this enum. A large number of lines had to be changed
because the enum values now live in their own namespace, but this is
arguably worth it since it is now much clearer what sort of constant
they are.

BUG=webrtc:5028

Review URL: https://codereview.webrtc.org/1424083002

Cr-Commit-Position: refs/heads/master@{#10449}

webrtc/modules/audio_coding/neteq/neteq_impl.h

@@ -111,7 +111,7 @@ class NetEqImpl : public webrtc::NetEq {
 
   // Associates |rtp_payload_type| with |codec| and stores the information in
   // the codec database. Returns kOK on success, kFail on failure.
-  int RegisterPayloadType(enum NetEqDecoder codec,
+  int RegisterPayloadType(NetEqDecoder codec,
                           uint8_t rtp_payload_type) override;
 
   // Provides an externally created decoder object |decoder| to insert in the
@@ -119,7 +119,7 @@ class NetEqImpl : public webrtc::NetEq {
   // associates it with |rtp_payload_type|. The decoder will produce samples
   // at the rate |sample_rate_hz|. Returns kOK on success, kFail on failure.
   int RegisterExternalDecoder(AudioDecoder* decoder,
-                              enum NetEqDecoder codec,
+                              NetEqDecoder codec,
                               uint8_t rtp_payload_type,
                               int sample_rate_hz) override;