Answer 1

Actually, designing a good layout can be harder than making very intense calculations. A simple solution is to learn HTML, CSS, JS and develop web apps. You can learn Android XML, Java if you want to develop apps for Android.

Answer 2

HTML

Answer 3

html,css,js nice and simple

Answer 4

Since you want to make Android applications then these are my recommendations:
Java with Android Studio
C# with Xamarin (or use Unity engines if you want to make games)