Guard thread safety of date formatting

The `SimpleDateFormat` format class is not thread safe. Without any type
of synchronization it would be possible for multiple threads to attempt
to format dates at the same time resulting in some odd dates. As the
formatter is a private constant this takes the approach of simply
synchronizing on it. This way we ensure only one thread is using it at a
time.

An alternative approach would be to create a thread local with each
thread having it's own dedicated formatter. I opted to not use this
approach because it is slightly more complicated and each thread would
pay the penalty of initial formatter creation. If it turns out that
there are only a small number of re-used threads which use this class
and the synchronization overhead is too large we can move to that
approach.

This fix is being omitted from the changelog as it does not affect any
external behavior - just potential debug logging information.

Author: cupakromer

src/main/java/org/altbeacon/beacon/service/Stats.java

@@ -14,6 +14,10 @@
 public class Stats {
     private static final Stats INSTANCE = new Stats();
     private static final String TAG = "Stats";
+
+    /**
+     * Synchronize all usage as this is not a thread safe class.
+     */
     private static final SimpleDateFormat SIMPLE_DATE_FORMAT = new SimpleDateFormat("HH:mm:ss.SSS");
 
     private ArrayList<Sample> mSamples;
@@ -103,7 +107,13 @@ private void logSample(Sample sample, boolean showHeader) {
     }
 
     private String formattedDate(Date d) {
-        return d == null ? "" : SIMPLE_DATE_FORMAT.format(d);
+        String formattedDate = "";
+        if (d != null) {
+            synchronized (SIMPLE_DATE_FORMAT) {
+                formattedDate = SIMPLE_DATE_FORMAT.format(d);
+            }
+        }
+        return formattedDate;
     }
 
     private void logSamples() {