Use correct pid when running under test harness

This is a partial fix which addresses one of the root causes of this bug,
but only when running under test harness.  It has no effect otherwise.

Bug: 8598539
Change-Id: I87444daef1d76b17544f331e651ba1c87893c381
diff --git a/services/audioflinger/ServiceUtilities.cpp b/services/audioflinger/ServiceUtilities.cpp
index 6a58852..d15bd04 100644
--- a/services/audioflinger/ServiceUtilities.cpp
+++ b/services/audioflinger/ServiceUtilities.cpp
@@ -21,8 +21,9 @@
 
 namespace android {
 
-// This optimization assumes mediaserver process doesn't fork, which it doesn't
-const pid_t getpid_cached = getpid();
+// Not valid until initialized by AudioFlinger constructor.  It would have to be
+// re-initialized if the process containing AudioFlinger service forks (which it doesn't).
+pid_t getpid_cached;
 
 bool recordingAllowed() {
     if (getpid_cached == IPCThreadState::self()->getCallingPid()) return true;