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/decoder_database.h

@@ -37,11 +37,10 @@ class DecoderDatabase {
   struct DecoderInfo {
     // Constructors.
     DecoderInfo()
-        : codec_type(kDecoderArbitrary),
+        : codec_type(NetEqDecoder::kDecoderArbitrary),
           fs_hz(8000),
           decoder(NULL),
-          external(false) {
-    }
+          external(false) {}
     DecoderInfo(NetEqDecoder ct, int fs, AudioDecoder* dec, bool ext)
         : codec_type(ct),
           fs_hz(fs),