Add ViewActions utilities for Android

I often display/hide views based on a conditional, especially for growth/onboarding. Here's one example:

```java
View findYourFriendsButton = // ...
Observable<Integer> getFriendsCount = // ...
getFriendsCount()
        .map(integer -> integer < 10)
        .subscribe(setVisibility(findYourFriendsButton));
```