Support EXTRA_DEVICE_PASSWORD_REQUIREMENT_ONLY

When set, only enforce password requirement explicitly set device-wide.

As part of the change, restructure the code such that ChooseLockGeneric
becomes the central place for aggregating password requirements from
different parties, while ChooseLockPassword only enforces whatever
password reuirement it is told (by ChooseLockGeneric via intent extras)

Bug: 169832516
Test: m RunSettingsRoboTests ROBOTEST_FILTER=com.android.settings.password

Change-Id: I0acbea4819c13d4a8444c7b06928baccead18837