Fix NPE in LockPatternUtils

mDevicePolicyManager field can be null if getDevicePolicyManager()
is never called. A previous refactor removed an apparent unused call
to getDevicePolicyManager(), leading to NPE when mDevicePolicyManager
is accessed. We should always use getDevicePolicyManager() to access
DevicePolicyManager instance.

Test: manual
Bug: 34612758
Change-Id: Ic964b4ee4e3c56301295b8f0629bd005c732c5c4