Deal with daylight savings time in battery chart.

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