Change MainActivity.java to MainActivity.kt

[DeFuex]

• Change java to kotlin
• Used kotlin standard functions to make code more readable.
• Added synthetic view injection

[YvesCheung]

I appreciate your contribution!!
There is a reason to write the demo in Java, not Kotlin:
See #5 and #9 , which means that Many developers still can't understand Kotlin and expect a Java version.
So I write a Java demo which may help Java users...

[DeFuex]

Thanks, now i wish i would understand Chinese :) -> #5 #9 ...i understand your point, but is there no value of not adding a secondary Activity showing users how to use your library in Kotlin?