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/AAudioServiceEndpointShared.cpp b/services/oboeservice/AAudioServiceEndpointShared.cpp
index 2f1ec7e..0a415fd 100644
--- a/services/oboeservice/AAudioServiceEndpointShared.cpp
+++ b/services/oboeservice/AAudioServiceEndpointShared.cpp
@@ -181,8 +181,8 @@
 // Get timestamp that was written by the real-time service thread, eg. mixer.
 aaudio_result_t AAudioServiceEndpointShared::getFreeRunningPosition(int64_t *positionFrames,
                                                                   int64_t *timeNanos) {
-    if (mAtomicTimestamp.isValid()) {
-        Timestamp timestamp = mAtomicTimestamp.read();
+    if (mAtomicEndpointTimestamp.isValid()) {
+        Timestamp timestamp = mAtomicEndpointTimestamp.read();
         *positionFrames = timestamp.getPosition();
         *timeNanos = timestamp.getNanoseconds();
         return AAUDIO_OK;