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