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

(cherry picked from commit 8d8ef00c)

Change-Id: If074a67fa52943c0ec7bb5c5bbe5a11f54fb1f97