UsageStats: Fix issue where initializing data for first time would cause crash

With the updated rolling window of stats for app idleness, we need to make sure
it is populated before we initialize some defaults.

Now that we look at older entries to figure out idleness, if those
entries are in the future (due to time change), set them to the current
screen on time.

Bug:26504153
Change-Id: I4c21e8f094a1a6727373368d043f523053280d7d