isDisplayingAtLeast does not check whether a parent obscures the view

### Description

According to the [documentation](https://developer.android.com/reference/android/support/test/espresso/matcher/ViewMatchers.html#isdisplayingatleast) isDisplayingAtLeast:

> Returns a matcher which accepts a view so long as a given percentage of that view's area **is not obscured by any parent view** and is thus visible to the user.

That's not what happens. It actuality the matcher only checks whether a sufficient portion of the view area is within the boundaries of the screen. It's evident from examining the code of `IsDisplayingAtLeastMatcher#matchesSafely`.

### Steps to Reproduce

Make the parent view clip a portion of the child view, e.g.:

```jsx

```

only 74% of the child view will be visible to the user, the bottom 26% will be clipped by the parent. Both views will render within the boundaries of the screen.

### Expected Results

`isDisplayingAtLeast(75)` should fail.

### Actual Results

`isDisplayingAtLeast(75)` passes even though only 74% is displayed.

### AndroidX Test and Android OS Versions

Android 9, espresso-core 3.1.1

Start at IsDisplayingAtLeastMatcher#matchesSafely and reproduce the supplied parent/child clipping case. Trace how the displayed area is calculated, then verify that isDisplayingAtLeast(75) fails when the parent clips 26% of the child while an unobscured case still passes.

Written by the indexing model from the issue text.