Use blog timezone settings when retrieving Stats

Fix #1220 
/cc @astralbodies 
I notice a lot of SimpleDateFormat objects being created in this class, which [can hurt performance](http://www.thedwick.com/2008/04/simpledateformat-performance-pig/). No need to change this in this PR, but if down the road you find stats isn't performing well, you can try using the same [ThreadLocal<DateFormat> optimization](https://github.com/wordpress-mobile/WordPress-Android/blob/develop/WordPress/src/main/java/org/wordpress/android/util/DateTimeUtils.java#L23) I added to DateTimeUtils, ex:
```
private static final ThreadLocal<SimpleDateFormat> StatsDateFormatDay = new ThreadLocal<SimpleDateFormat>() {
    @Override
    protected SimpleDateFormat initialValue() {
        return new SimpleDateFormat("yyyy-MM-dd");
    }
};

public static String getYesterdaysDateTZ(String blogTimeZoneOption) {
    SimpleDateFormat gmtDf = StatsDateFormatDay.get();
    String todayDateTZ = getCurrentDateTZ(blogTimeZoneOption);
    long yesterdayMillis = StatsUtils.toMs(todayDateTZ);
    return gmtDf.format(new Date(yesterdayMillis - StatsUtils.ONE_DAY));
}
```