aaudio: cleanup code that reads timing model

Only use the clock model for the position when
it is running and when the service is not updating
the FIFO counter.

Bug: 120932593
Bug: 122374244
Test: test_return_stop
Test: test_return_stop -i
Change-Id: I6d6a23b35dca5529b6221e1b3b4a4e6672093bf2
diff --git a/media/libaaudio/src/client/IsochronousClockModel.h b/media/libaaudio/src/client/IsochronousClockModel.h
index 7182376..46ca48e 100644
--- a/media/libaaudio/src/client/IsochronousClockModel.h
+++ b/media/libaaudio/src/client/IsochronousClockModel.h
@@ -36,7 +36,15 @@
     void start(int64_t nanoTime);
     void stop(int64_t nanoTime);
 
-    bool isStarting();
+    /**
+     * @return true if the model is starting up
+     */
+    bool isStarting() const;
+
+    /**
+     * @return true if the model is running and producing valid results
+     */
+    bool isRunning() const;
 
     void processTimestamp(int64_t framePosition, int64_t nanoTime);