DeskClock: Fix the AM/PM on time picker dialog display uncomplete

TimePicker use a fix font size compute the hours and minutes numbers width
But the label font size can set by app, so it will compute error width.
lead to the am/pm position error.

When app use customer fontsize, use this size compute the time label
size.

Change-Id: Ib42983684439f25ecc84940424048c09b3069e39
CRs-fixed: 783871