Fix NPE in DateFormat.is24HourFormat.

In some cases, we end up being called by code that doesn't have a valid
Context. It got away with this historically because it wasn't formatting
times (just dates), so it never went down a path that tried to query the
user's 12/24-hour preference. This patch just ensures that we don't try
to get the preference unless we actually need it.

Bug: 10339015
Change-Id: I2df466d85cdeba14dbf882498808cbad9bbb57f4