KEYMAPPER: Add DPAD_CENTER as hardware input source

And define an event for it as a JoystickButton

Some controllers produce this key press with their DPAD, typically to select and interact with options.
See: https://developer.android.com/develop/ui/views/touch-and-input/game-controllers/controller-input
Fire TV remote controller is such a contoller.
see: https://developer.amazon.com/docs/fire-tv/remote-input.html

backends/keymapper/hardware-input.cpp

@@ -267,6 +267,7 @@ const HardwareInputTableEntry defaultJoystickButtons[] = {
 	{ "JOY_DOWN",           JOYSTICK_BUTTON_DPAD_DOWN,      _s("D-pad Down")     },
 	{ "JOY_LEFT",           JOYSTICK_BUTTON_DPAD_LEFT,      _s("D-pad Left")     },
 	{ "JOY_RIGHT",          JOYSTICK_BUTTON_DPAD_RIGHT,     _s("D-pad Right")    },
+	{ "JOY_CENTER",         JOYSTICK_BUTTON_DPAD_CENTER,    _s("D-pad Center")   },
 	{ nullptr,              0,                              nullptr              }
 };