Move AudioDecoder and AudioDecoderFactory mocks to webrtc/test/

AudioDecoder and AudioDecoderFactory are in webrtc/api/ now, so move
their mocks to someplace central where tests from all over WebRTC are
allowed to #include them.

BUG=webrtc:5805

Review-Url: https://codereview.webrtc.org/2798063004
Cr-Commit-Position: refs/heads/master@{#17619}
diff --git a/webrtc/modules/audio_coding/BUILD.gn b/webrtc/modules/audio_coding/BUILD.gn
index 6d14223..9c7e5ee 100644
--- a/webrtc/modules/audio_coding/BUILD.gn
+++ b/webrtc/modules/audio_coding/BUILD.gn
@@ -2064,7 +2064,6 @@
       "neteq/dtmf_tone_generator_unittest.cc",
       "neteq/expand_unittest.cc",
       "neteq/merge_unittest.cc",
-      "neteq/mock/mock_audio_decoder.h",
       "neteq/mock/mock_buffer_level_filter.h",
       "neteq/mock/mock_decoder_database.h",
       "neteq/mock/mock_delay_manager.h",
@@ -2126,6 +2125,7 @@
       "../../base:rtc_base_tests_utils",
       "../../common_audio",
       "../../system_wrappers:system_wrappers",
+      "../../test:audio_codec_mocks",
       "../../test:field_trial",
       "../../test:rtp_test_utils",
       "../../test:test_common",