Mainmenu: don't show change keys dialog on android

The change keys dialog can't be left. It doesn't make
much sense to show it on Android in the first place,
therefore disable it, just like commit

aed70cb0 'Disable sound and key binding settings in "pause" menu on android'

has disabled it for the esc menu.

Fixes #4115.

builtin/mainmenu/tab_settings.lua

@@ -203,9 +203,15 @@ local function formspec(tabview, name, tabdata)
 		"checkbox[8,0;cb_shaders;" .. fgettext("Shaders") .. ";"
 				.. dump(core.setting_getbool("enable_shaders")) .. "]"
 
+	if PLATFORM ~= "Android" then
+		tab_string = tab_string ..
+			"button[8,4.75;3.75,0.5;btn_change_keys;"
+			.. fgettext("Change keys") .. "]"
+	end
+
 	tab_string = tab_string ..
-		"button[8,4.75;3.75,0.5;btn_change_keys;" .. fgettext("Change keys") .. "]" ..
-		"button[0,4.75;3.75,0.5;btn_advanced_settings;" .. fgettext("Advanced Settings") .. "]"
+		"button[0,4.75;3.75,0.5;btn_advanced_settings;"
+		.. fgettext("Advanced Settings") .. "]"
 
 
 	if core.setting_get("touchscreen_threshold") ~= nil then