Fix 2463886: Allow delayed fetching of DevicePolicyManager in LockPatternUtils.

Today we're seeing a crash that's likely caused by a change in the order in which
system services start.

The crash we're seeing happens in response to user interaction which happens after the
boot process completes, so we should re-fetch the DevicePolicyManager if we weren't
able to get it when LockPatternUtils was constructed.