Lock screen without disabling fingerprint
Notice how the apex launcher or nova launcher got it way around. screen lock through apex launcher or nova launcher doesn't disable fingerprint scanner!
What it actually does is that it sets the screen to complete black i.e. soft lock (which gives user a false impression of screen off) and then after 5-10 seconds the display actually goes off due to time-out.
other app which implemented the same functionality OFF+ (Screen Off / Fingerprint unlock support).（I guess it modifys "lock screen timeout"）
If the phone is rooted then a shell command can be executed to simulate the power key su input keyevent 26 but there might be a fraction of delay.
well, if some root user is looking, there's also a xposed module which solves this issue FingerprintEnabler
That is the normal flow. It takes the device to restarted state where the device will ask for a pattern or password. Google have not provided any api for this. You can check any app in PlayStore which has locking feature will behave like this.
Edited: For the below queries you have asked response is shared here
How can a user lock his phone without using the power button (through an app for example) and still be able to unlock it using fingerprint?
There's intentionally no API for apps to call to lock the device.
Is there a security reason/concern around this?
Yes, there's a DOS (denial of service) concern if we allow apps to lock the device. Either intentional or accidental (app bug).
Found it here[https://code.google.com/p/android/issues/detail?id=79735#c115]
I think you need to search for how to lock with app shortcut and not with power button. Atleast I could help to this level.
I would like to add some points to the JerryGoyal's excellent answer.
Below is the screenshot from Microsoft Launcher which shows the two ways to lock the screen. It clearly states that the method via Device Policy Manager will not work with fingerprint.
A work around is done by showing a black screen until the screen times out.
In the next screen it asks for permission to Modify system settings to alter the screen timeout period.
