Merge branch 'not-null-by-default' into 'master'

Null safety annotations

The @NotNullByDefault annotation marks all fields, methods and parameters in a class or package @NotNull, so Android Studio will warn if values that may be null are used. Please use this annotation for new classes, and specify @Nullable for any fields, methods and parameters that may be null.

Injected fields are initialised to null, so injected classes should use @MethodsNotNullByDefault and @ParametersNotNullByDefault, or specify @Nullable for injected fields.

See merge request !349