Bug 1207478: P1. Prevent microseconds calculation overflow. r=gerald

This leaves us safe until year 294441 (assuming unix timestamp).

media/libstagefright/binding/include/mp4_demuxer/MoofParser.h

@@ -35,7 +35,9 @@ public:
 
   Microseconds ToMicroseconds(int64_t aTimescaleUnits)
   {
-    return aTimescaleUnits * 1000000ll / mTimescale;
+    int64_t major = aTimescaleUnits / mTimescale;
+    int64_t remainder = aTimescaleUnits % mTimescale;
+    return major * 1000000ll + remainder * 1000000ll / mTimescale;
   }
 
   uint64_t mCreationTime;