Answer 1

You should have some knowlegde in object oriented programming in Java,and you can begin with android.Because for building GUI in Android you don't have to know GUI in Java with swing or JavaFx....Android uses a very easy XML for that.So,just lean Object oriented and then move to Android dev.
Some useful links:

Basics in Java: https://www.coursera.org/learn/initiation-programmation-java
https://www.udacity.com/course/java-programming-basics--ud282

Java datastructures: https://www.coursera.org/learn/java-programming-arrays-lists-data
Learn OOP in Java: https://www.coursera.org/learn/object-oriented-java
https://www.coursera.org/learn/programmation-orientee-objet-java

Solelearn java tutorial: https://www.sololearn.com/Course/Java/

Android dev basics: https://www.udacity.com/course/new-android-fundamentals--ud851

https://www.udacity.com/course/android-basics-user-input--ud836

Answer 2

Java/ Kotlin for Android Studio.
C# for Xamarin.

Edit: I'd recommend Java with Android Studio.

Android resources:
https://www.sololearn.com/discuss/823823/?ref=app

Answer 3

You might want to check out the today's free ebook on packtpub - it's about Kotlin, one of the two native languages for Android development:
https://www.sololearn.com/discuss/447092/?ref=app

Answer 4

learn java