Can't attach Android Studio's debugger to Android process

So finally I got it working, by:

Checking 'Show all processes' checkbox!

BUT as you see in the question, checkbox was on at the beginning, so there were multiple issues.

To summarize, these are other actions to be done:

• Kill all adb processes (use ps -x | grep adb and kill -9 [process id])
• adb start-server in terminal
• Reconnect device
• adb devices (to make sure device is connected successfully)
• Make sure Android DDMS Devices | logcat shows your app and only one device is listed.
• Check Show all processes checkbox. See screenshot

I solved this way

a. Closed Android Studio
b. Did adb kill-server from a terminal. Also check the adb process in also killed from the Running processes window in PC
c. Started Android Studio
d. Android Studio tried to initialize adb again and back to work.

It could be because the release build variant (which you can set in the Build-Variants section) has been chosen mistakenly, only debug-variant is debuggable.

Hope this can be helpful to some friends.