Add histograms for FastMixer

Bug: 37153050
Test: dumpsys media.log shows histograms for audio activity on FastMixer
Change-Id: I6bb4bf19d62da1ed1d8cda43d428b03153a35de3
diff --git a/services/audioflinger/FastThread.cpp b/services/audioflinger/FastThread.cpp
index a0bf63b..ae4747e 100644
--- a/services/audioflinger/FastThread.cpp
+++ b/services/audioflinger/FastThread.cpp
@@ -91,6 +91,9 @@
 
 bool FastThread::threadLoop()
 {
+    // LOGT now works even if logWriterTLS is nullptr, but we're considering changing that,
+    // so this initialization permits a future change to remove the check for nullptr.
+    logWriterTLS = &mDummyNBLogWriter;
     for (;;) {
 
         // either nanosleep, sched_yield, or busy wait