Stop recognizing legacy password hashes

LockSettingsService supports maintaining a password history, to support
DevicePolicyManager#setPasswordHistoryLength().  The original
implementation of this feature used an insecure password hash function
which effectively leaked passwords to DE storage.  This was fixed in
Android 9 (b/32826058), but the ability to recognize the insecure legacy
password hashes was retained to make any password reuse requirement be
enforced continuously as devices were upgraded.

However, because Android 9 was so long ago, it's no longer useful to
still recognize these legacy hashes.  Even if, hypothetically, a device
were to be upgraded all the way from a version *before* Android 9 to the
current version, dropping support for the legacy hashes would just mean
that passwords used before Android 9 could be reused.

We shouldn't keep around known-insecure code, since it could be
accidentally misused in the future.

Thus, let's begin the process of removing this code. Stop recognizing
legacy hashes in the password history.

Bug: 442877927
Flag: android.security.stop_recognizing_legacy_password_hashes
Test: atest FrameworksServicesTests:com.android.server.locksettings
Change-Id: I260b1f05ef6779d4df4f88ad686c34dc394faf58