Bypass unnecessary resampling.

This change keeps the original 48 kHz signal and uses it for the
fullband processing given that the following requirements are
fulfilled:
- Input signal is 48 kHz
- Output signal is 48 kHz
- Multiband processing is performed at 32 kHz
- The multiband processing does not modify the original signal
This avoids unnecessary, lossy resampling and band merging.

Bug: b/130016532
Change-Id: I690c26faba07eab0cbff6c0a95a81d89255dd1a1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/155966
Commit-Queue: Gustaf Ullberg <gustaf@webrtc.org>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29425}

modules/audio_processing/include/mock_audio_processing.h

@@ -89,6 +89,7 @@ class MockEchoControl : public EchoControl {
                void(AudioBuffer* capture, bool echo_path_change));
   MOCK_CONST_METHOD0(GetMetrics, Metrics());
   MOCK_METHOD1(SetAudioBufferDelay, void(size_t delay_ms));
+  MOCK_CONST_METHOD0(ActiveProcessing, bool());
 };
 
 class MockAudioProcessing : public ::testing::NiceMock<AudioProcessing> {