New @TestApi: DevicePolicyManager.isNewUserDisclaimerAcknowledged()

It also changes acknowledgeNewUserDisclaimer() to allow callers with
INTERACT_ACROSS_USERS permissions - that's more consistent with other
methods and allows it to be called by tests.

Test: atest com.android.cts.devicepolicy.DeviceOwnerTest#testCreateAndManageUser_newUserDisclaimer
Test: m update-api

Bug: 203822627
Bug: 215375992

Change-Id: I8b98a1af60db65c1b27f19cf3e93b21d33d88278
Merged-In: I8b98a1af60db65c1b27f19cf3e93b21d33d88278
(cherry picked from commit 9a6b808d)