Fixes some DPM APIs to use the context user.

This APIs are primarily used by CarService, which always runs as
system user, so they cannot rely on the calling user id.

Test: atest CtsDevicePolicyManagerTestCases com.android.cts.devicepolicy.DeviceOwnerTest#testCreateAndManageUser_newUserDisclaimer # on automotive and phone

Test: manual verification

Change-Id: I56bb8cd12be766ef9da6c71a524912f7f8680aad