Add getExternalPosition in MMapStreamInterface.

GetExternalPosition will return a recent count of the number of
audio frames presented/received to/from an external observer. Using
the new API can return a precise timestamp to client, which can
help on A/V sync.

Test: atest AAudioTests
Bug: 158609200
Change-Id: If74e1d859a2bb2b4dba6af0e4e9164f05d19d962
diff --git a/services/oboeservice/AAudioServiceEndpointMMAP.h b/services/oboeservice/AAudioServiceEndpointMMAP.h
index b6003b6..a2a0922 100644
--- a/services/oboeservice/AAudioServiceEndpointMMAP.h
+++ b/services/oboeservice/AAudioServiceEndpointMMAP.h
@@ -85,6 +85,8 @@
         return mHardwareTimeOffsetNanos;
     }
 
+    aaudio_result_t getExternalPosition(uint64_t *positionFrames, int64_t *timeNanos);
+
 private:
     MonotonicCounter                          mFramesTransferred;
 
@@ -101,6 +103,8 @@
 
     int64_t                                   mHardwareTimeOffsetNanos = 0; // TODO get from HAL
 
+    bool                                      mExternalPositionSupported = true;
+
 };
 
 } /* namespace aaudio */