Deal with daylight savings time in battery chart.

Without daylight saving time the middle between two beginnings of days
can only be another beginning of a day or noon. Due to daylight savings
time this can be shifted by up to one hour in either direction.

This change corrects the middle value.

Testing Done: Tried the calculation of the three values with Lisbon time
and

mStartWallTime = 1414254027000L;
mEndWallTime = 1414473627000L;

which spans over 10/26/2014, the day daylight saving ended in Lisbon.

Change-Id: I2d2ba6e1fd2e409ca3f6b0f7674ec250da7ffad5

src/com/android/settings/fuelgauge/BatteryHistoryChart.java

@@ -1118,8 +1118,13 @@ public class BatteryHistoryChart extends View {
                 if (startRoundTime < endRoundTime) {
                     addDateLabel(calStart, mLevelLeft, mLevelRight, isDayFirst);
                     Calendar calMid = Calendar.getInstance();
-                    calMid.setTimeInMillis(startRoundTime + ((endRoundTime - startRoundTime) / 2));
+
+                    // The middle between two beginnings of days can be anywhere between -1 to 13
+                    // after the beginning of the "median" day.
+                    calMid.setTimeInMillis(startRoundTime + ((endRoundTime - startRoundTime) / 2)
+                                           + 2 * 60 * 60 * 1000);
                     calMid.set(Calendar.HOUR_OF_DAY, 0);
+                    calMid.set(Calendar.MINUTE, 0);
                     long calMidMillis = calMid.getTimeInMillis();
                     if (calMidMillis > startRoundTime && calMidMillis < endRoundTime) {
                         addDateLabel(calMid, mLevelLeft, mLevelRight, isDayFirst);