wallpapers/potd: work around timezone issue

For people who live in a country that has more than one timezone, e.g. the United States, Bing will always update its picture at 12:00 a.m. Pacific Time (UTC-8), which is actually problematic for people living in America where the timezone is later than Pacific Time, because at 12:00 a.m. ET/CT/MT Bing in America still doesn't update the picture.

In those countries, the maximum update interval will be 1 hour, which is also used before 5.26.

BUG: 463345 FIXED-IN: 5.27