Fixed default Android D-PAD mapping so the back button is treated as B

src/joystick/SDL_gamecontroller.c

@@ -923,7 +923,7 @@ static ControllerMapping_t *SDL_PrivateGetControllerMapping(int device_index)
     if (!mapping && SDL_SYS_IsDPAD_DeviceIndex(device_index)) {
         SDL_bool existing;
         char mapping_string[1024];
-        SDL_snprintf(mapping_string, sizeof(mapping_string), "none,%s,a:b0,b:b1,dpdown:b12,dpleft:b13,dpright:b14,dpup:b11,", name);
+        SDL_snprintf(mapping_string, sizeof(mapping_string), "none,%s,a:b0,b:b4,dpdown:b12,dpleft:b13,dpright:b14,dpup:b11,", name);
         mapping = SDL_PrivateAddMappingForGUID(guid, mapping_string,
                           &existing, SDL_CONTROLLER_MAPPING_PRIORITY_DEFAULT);
     }