Fix inhibit suspend on Gnome

The inhibit dbus call to gnome sessionmanager expects flags as input. Currently the flags are set to 8, which will "Inhibit the session being marked as idle". This is insufficient to prevent the device from suspending while playing. We should also add 4 = "Inhibit suspending the session or computer". Hence, the updated flags to 12.

This change was tested on phosh/phoc and it properly prevents the system from suspending. The flags = 12 settings is also what gnome music players like Lollypop are using (as checked through the gnome sessionmanager dbus interface).