iSAC encoder: Make it possible to change target bitrate at any time

Not just at construction time.

Bug: webrtc:11704
Change-Id: I952c7dbe20774cc976065c7d2f992a80074ebf63
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/177663
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31550}

modules/audio_coding/BUILD.gn

@@ -386,6 +386,8 @@ rtc_source_set("isac_common") {
     "../../api/units:time_delta",
     "../../rtc_base:checks",
     "../../rtc_base:rtc_base_approved",
+    "../../rtc_base:safe_minmax",
+    "../../system_wrappers:field_trial",
   ]
   absl_deps = [ "//third_party/abseil-cpp/absl/types:optional" ]
 }