Refresh strong auth alarm when admin changes policy

When admin sets a new strong auth timeout policy, replace the existing
alarm (which enforces strong auth after the timeout) with a new one
with updated timeout.

Bug: 146188984
Test: atest com.android.server.locksettings.LockSettingsStrongAuthTest
Test: atest MixedManagedProfileTest#testRequiredStrongAuthTimeout
Change-Id: Ibcc13eb0d66697aff44192769b8fd817ca6800b8