Steps to reproduce:
1) Open the attached project.
2) Build and Run on iOS or Android.
- Make sure sound is enabled on the device.
3) Double-click the lock button.

Expected: the phone is locked and the music stops playing.
Actual: the phone is locked, but background music doesn't stop. The music only stops when the screen turns off.

On Android, if double clicking the lock button is assigned to open the camera, the phone might become stuck on the lock screen (trying to unlock the phone displays a black screen and locks it again automatically).

Reproduced on:
2018.3.0a8, 2017.4.4f1, 2017.4.3f1, 2017.1.5f1

Devices:
iPhone 8 Plus iOS 12.0
iPhone 7 iOS 11.4.1
Samsung Galaxy S9 Android 8.0.0