Make sure that timestamp does not go backward in MP4 file writer

Change-Id: I90745b9df7f19d61f3ab826bf9d2419fe788554e
diff --git a/media/libstagefright/MPEG4Writer.cpp b/media/libstagefright/MPEG4Writer.cpp
index e36d9fe..a15b84e 100644
--- a/media/libstagefright/MPEG4Writer.cpp
+++ b/media/libstagefright/MPEG4Writer.cpp
@@ -1569,9 +1569,9 @@
                 // The idea here is to avoid having two or more samples with the
                 // same timestamp in the output file.
                 if (mTimeScale >= 1000000LL) {
-                    timestampUs += 1;
+                    timestampUs = lastTimestampUs + 1;
                 } else {
-                    timestampUs += (1000000LL + (mTimeScale >> 1)) / mTimeScale;
+                    timestampUs = lastTimestampUs + (1000000LL + (mTimeScale >> 1)) / mTimeScale;
                 }
 #endif
             }