avoid multiple-"0" to exceed the length of upper bound

Limits number of "0"s not to exceed length of maximum number allowed
for the NumberPicker.

Steps to reproduce:
1. Settings -> Date & time -> uncheck "Automatic date & time"
2. Select "Set time"
3. Select Time Area on Set time dialog
4. insert 00000000 via NumberPicker

Bug: 8073759
Change-Id: I0f3f5303d9a4b559217adb436f244407a23e58c0