To reproduce:

1. Create new Unity project.
2. Build and run for any Sony Xperia device with built in camera button.
3. When app is loaded, press and hold built-in camera button.

Expected result: Unity application goes into background and built-in Camera opens up.
Actual result: Built in camera button does nothing.

Reproduced with 2017.3.0a7, 2017.2.0b11, 2017.1.1p2, 5.6.3p3, 5.5.4p5.

Devices under test:

Sony D6603 (Xperia Z3)*, OS:6.0.1, CPU:armeabi-v7a, GPU:Adreno (TM) 330,
Build:Sony/D6603/D6603:6.0.1/23.5.A.1.291/2769308465:user/release-keys

Notes: Only games made with Unity engine lacks this functionality. Any non-Unity games respond fine to built in camera button. Tested games : Asphalt 8, Angry Birds Friends, Dragon Ball Z Dokkan Battle. All of them work fine with integrated camera button and are availabe on Google play. If it is not intended, an optional checkbox should be added for enabling this feature.

Verified with: 2018.1.0a1