Convert various const strings to constexprs: chromecast/

This makes it possible to supply them to absl::StrFormat() as format
strings, and may enable better codegen.

Bug: 1371963
Change-Id: Ia34266e822b44ce11887fc478c54495527fa830f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4943408
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1210656}

chromecast/media/cma/backend/mixer/post_processors/governor_unittest.cc

@@ -22,7 +22,7 @@ namespace post_processor_test {
 
 namespace {
 
-const char* kConfigTemplate =
+constexpr char kConfigTemplate[] =
     R"config({"onset_volume": %f, "clamp_multiplier": %f})config";
 
 const float kDefaultClamp = 0.6f;