Split iSAC encoder/decoder: Test more cases (and make sure they work)

This patch tests separate iSAC encoder and decoder in more cases (32
kHz in addition to 16 kHz, and 30 ms adaptive and 60 ms nonadaptive).

In order to handle 32 kHz adaptive, the decoder needs to be told of
the encoder's sample rate (16 kHz worked already because that's the
default). And since we can't set the encoder's frame size without also
setting its bit rate, we need a way to set the decoder's bit rate as
well.

It turned out to be way too messy to continue verifying that the
bandwidth estimator does something reasonable in all these cases,
because it seems it doesn't. So the GetSetBandwidthInfo is now just
responsible for ensuring that split encoder/decoder behaves the same
as conjoined encoder/decoder; the job of verifying that the bandwidth
estimator does its job properly falls on some other test (that doesn't
exist yet).

Review URL: https://codereview.webrtc.org/1225093005

Cr-Commit-Position: refs/heads/master@{#9583}

webrtc/modules/audio_coding/codecs/isac/fix/source/isacfix.c

@@ -1110,6 +1110,13 @@ int16_t WebRtcIsacfix_Control(ISACFIX_MainStruct *ISAC_main_inst,
   return 0;
 }
 
+void WebRtcIsacfix_SetInitialBweBottleneck(ISACFIX_MainStruct* ISAC_main_inst,
+                                           int bottleneck_bits_per_second) {
+  ISACFIX_SubStruct* inst = (ISACFIX_SubStruct*)ISAC_main_inst;
+  assert(bottleneck_bits_per_second >= 10000 &&
+         bottleneck_bits_per_second <= 32000);
+  inst->bwestimator_obj.sendBwAvg = ((uint32_t)bottleneck_bits_per_second) << 7;
+}
 
 /****************************************************************************
  * WebRtcIsacfix_ControlBwe(...)