aaudio: rename timestamp variables

There were several different timestamp variables with slightly
different meaning but with the same name.
That made it hard to debug the code.

Test: atest CtsNativeMediaAAudioTestCases
Change-Id: Iebf438676925c698377abe02c81f3d559829318a
diff --git a/services/oboeservice/AAudioServiceEndpoint.h b/services/oboeservice/AAudioServiceEndpoint.h
index 43b0a37..3616fa2 100644
--- a/services/oboeservice/AAudioServiceEndpoint.h
+++ b/services/oboeservice/AAudioServiceEndpoint.h
@@ -121,7 +121,7 @@
     mutable std::mutex       mLockStreams;
     std::vector<android::sp<AAudioServiceStreamBase>> mRegisteredStreams;
 
-    SimpleDoubleBuffer<Timestamp>  mAtomicTimestamp;
+    SimpleDoubleBuffer<Timestamp>  mAtomicEndpointTimestamp;
 
     android::AudioClient     mMmapClient;   // set in open, used in open and startStream