Fix missing @NonNull annotations

[wmontwe]

Some of the Java files are lacking @NonNull annotations, even when declared by Android already. We should fix that to reduce the ammount of false nullable warnings and ease the Java to Kotlin conversion.

[cketti]

It's probably not worth the effort of going through the Java code just for that. It is a good idea to add the annotations right before doing a conversion to Kotlin, though.

[wmontwe]

Android Studio / Intellij Idea have good code analyis tools that could do most of the job already.