Just to clarify, implementing a lock screen has no relation to encrypting the file system, they're two different security aspects that apply to two separate things that are independent of each other.
-- A lock screen is a way of securing your phone from other people being able to access your phone when it gets restarted or woken from sleep mode. It makes no difference in any way if the internal storage media is encrypted or not encrypted and you do or don't have lock screen enabled. Adding a lock screen (passcode, swipe, fingerprint, etc.) does not encrypt nor decrypt the internal storage media, its purpose is to keep other people from readily logging into your phone so they don't have easy access to all the things stored in it.
-- When the internal storage media is encrypted, any data that gets stored in it is protected by a cryptographic key. That encryption key is unique (only works on the device it was created on), and gets stored locally (only on the device it was created on). Encryption relates to the storage media, it has no bearing on if a lock screen is or isn't set up. Early versions of Android had no ability to encrypt the storage media, around KitKat or maybe even earlier some manufacturers were allowing it, but it wasn't until Lollipop or maybe Marshmallow (???) before encrypting the storage media became the default for all Android devices.
