Make PasswordMetrics#computeForPasswordOrPin() private

PasswordMetrics#computeForPasswordOrPin() has unnecessarily broad
visibility and was being misused by the clockwork settings app (fixed by
http://ag/23923392).  Now it is only used by PasswordMetrics itself and
unit tests, but the unit tests can just use computeForCredential()
instead.  Do that, and make computeForPasswordOrPin() a private method.

Test: atest PasswordMetricsTest com.android.server.devicepolicy.DevicePolicyManagerTest
Change-Id: I1028e946ac97bd51c489e630d2de6df7685d454f