Load DevicePolicyManager only when needed in LockSettingsService

Since DevicePolicyManager is published later than LockSettingsService,
we shouldn't be getting DevicePolicyManager when LockSettingsService is
being constructed.

Bug: 36424241
Test: cts-tradefed run cts-dev -m CtsDevicePolicyManagerTestCases -t com.android.cts.devicepolicy.MixedProfileOwnerTest#testResetPasswordWithToken
Change-Id: I5e9da78e4fe802c70d34dc3c9dae4f2677ed631d