Use ATRACE macros instead of Tracer statics

ATRACE_BEGIN and ATRACE_END have replaced the static
Tracer::traceBegin and Tracer::traceEnd functions, so
use them instead.

Fixes compilation errors when tracing is enabled.

Change-Id: I4d1147d2f76afcdf113e9986f0544cb848802b15
diff --git a/services/audioflinger/FastMixer.cpp b/services/audioflinger/FastMixer.cpp
index 3c8a256..2160ea3 100644
--- a/services/audioflinger/FastMixer.cpp
+++ b/services/audioflinger/FastMixer.cpp
@@ -425,11 +425,11 @@
             //       but this code should be modified to handle both non-blocking and blocking sinks
             dumpState->mWriteSequence++;
 #if defined(ATRACE_TAG) && (ATRACE_TAG != ATRACE_TAG_NEVER)
-            Tracer::traceBegin(ATRACE_TAG, "write");
+            ATRACE_BEGIN("write");
 #endif
             ssize_t framesWritten = outputSink->write(mixBuffer, frameCount);
 #if defined(ATRACE_TAG) && (ATRACE_TAG != ATRACE_TAG_NEVER)
-            Tracer::traceEnd(ATRACE_TAG);
+            ATRACE_END();
 #endif
             dumpState->mWriteSequence++;
             if (framesWritten >= 0) {