CDK BreapointObserver emits multiple 'true' for differents sizes of screen

#### Reproduction
Steps to reproduce:
1. Write this code in a service, then subscribe. See in console.
```
isLandscape$: Observable = this.breakpointObserver.observe(['(orientation: portrait)', '(orientation: landscape)']).pipe(
map((result: BreakpointState) => {
console.log('result', result);
return result.matches;
})
);
screenSizes$: Observable = this.breakpointObserver
.observe([Breakpoints.XSmall, Breakpoints.Small, Breakpoints.Medium, Breakpoints.Large, Breakpoints.XLarge])
.pipe(
map((result: BreakpointState) => {
console.log('result', result);
return result.matches;
})
);
```

#### Expected Behavior

What behavior were you expecting to see?

I'm expecting to get only 1 condition set to true for media queries defined in the CDK.

#### Actual Behavior

What behavior did you actually see?

My browser is 1903px wide. The 1st time i load the page, I see in the console the Large and XLarge conditions set to true:

```
(min-width: 1280px) and (max-width: 1919.99px): true
(min-width: 1920px): true
```
Then, when I resize my window, this time, the CDK emits only `(min-width: 1280px) and (max-width: 1919.99px): true`.

The same type of error occurs screen when I first get to 'xs' screen by resizing the window: 2 conditions set to true (for Small & XSmall screen sizes):

```
(max-width: 599.99px): true
(min-width: 600px) and (max-width: 959.99px): true
```
Then, when I resize again my window, i only get `(max-width: 599.99px): true`.

ALSO:

When i reload the page in 'xs' size, I get the right values set for both isLandscape & Screen size.
But when i resize the page, I get:
```
breakpoints: {
(orientation: landscape): false
(orientation: portrait): false
}
matches: false
```
I also get a no match from times to times for the screen sizes...

So, that's just not possible.
- The CDK should be able to emit that I'm still in portrait mode,
- the CDK should be able to detect my screen size any time.

#### Environment

- Angular: 8.0.0
- CDK/Material: 8.0.0
- Browser(s): Chrome 76
- Operating System (e.g. Windows, macOS, Ubuntu): Windows 10

从 reproduction 中展示的 BreakpointObserver 用法开始，并在 Angular 8 中使用报告的 viewport 尺寸运行它。比较方向和屏幕尺寸查询的初始发射结果与调整大小时的发射结果。完成标准是每个 viewport 都能持续报告正确的匹配条件，且不会出现暂时性的重复结果或无匹配结果。

由索引模型根据 Issue 内容生成。