DPM: Separate storage of PasswordMetrics from other state

Keep the PasswordMetrics for each user on a separate map from the rest
of the profile data (kept in the DevicePolicyData object).

The PasswordMetrics are not persisted to disk, unlike other fields of
DevicePolicyData (to avoid making it easy for an attacker to brute-force
the password).

Additionally, and the cause of the bug mentioned below, the
PasswordMetrics should not be cleared when a user is started, but
persisted.

Bug: 73899116
Test: Manual with TestDPC
Test: atest FrameworksServicesTests:DevicePolicyManagerTest
Test: runtest -c com.android.server.devicepolicy.DevicePolicyManagerTest frameworks-services
Change-Id: Id42145665f9ff477ea67fe44e8e55fc6586b8edf