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/Threads.h b/services/audioflinger/Threads.h
index cc66cad..ff7bb63 100644
--- a/services/audioflinger/Threads.h
+++ b/services/audioflinger/Threads.h
@@ -976,7 +976,7 @@
     sp<NBAIO_Source>        mTeeSource;
 #endif
     uint32_t                mScreenState;   // cached copy of gScreenState
-    static const size_t     kFastMixerLogSize = 4 * 1024;
+    static const size_t     kFastMixerLogSize = 8 * 1024;
     sp<NBLog::Writer>       mFastMixerNBLogWriter;