Moved legacy_encoded_audio_frame into audio_decoder_interface.

audio_decoder.cc depends on LegacyEncodedAudioFrame and
LegacyEncodedAudioFrame depends on AudioDecoder::EncodedAudioFrame, so
there's no clear way to separate them as of now. This error is also
hodling up builds downstream. I expect we'll revisit these
dependencies as part of the upcoming larger restructuring effort.

NOTRY=true
BUG=webrtc:5805

Review-Url: https://codereview.webrtc.org/2359763002
Cr-Commit-Position: refs/heads/master@{#14329}

webrtc/modules/audio_coding/codecs/g711/g711.gypi

@@ -14,7 +14,6 @@
       'dependencies': [
         'audio_encoder_interface',
         'audio_decoder_interface',
-        'legacy_encoded_audio_frame',
       ],
       'sources': [
         'audio_decoder_pcm.cc',

webrtc/modules/audio_coding/codecs/g722/g722.gypi

@@ -13,7 +13,6 @@
       'dependencies': [
         'audio_encoder_interface',
         'audio_decoder_interface',
-        'legacy_encoded_audio_frame',
       ],
       'sources': [
         'audio_decoder_g722.cc',

webrtc/modules/audio_coding/codecs/interfaces.gypi

@@ -14,6 +14,8 @@
       'sources': [
         'audio_decoder.cc',
         'audio_decoder.h',
+        'legacy_encoded_audio_frame.cc',
+        'legacy_encoded_audio_frame.h',
       ],
       'dependencies': [
         '<(webrtc_root)/base/base.gyp:rtc_base_approved',

webrtc/modules/audio_coding/codecs/pcm16b/pcm16b.gypi

@@ -14,7 +14,6 @@
       'dependencies': [
         'audio_encoder_interface',
         'audio_decoder_interface',
-        'legacy_encoded_audio_frame',
         'g711',
       ],
       'sources': [