CDK Platform always return true for ANDROID for all platform

`"@angular/cdk": "9.2.4"`

```
console.log('PLATFORMS');
console.log(this.platform.ANDROID);
console.log(this.platform.BLINK);
console.log(this.platform.EDGE);
console.log(this.platform.FIREFOX);
console.log(this.platform.IOS);
console.log(this.platform.isBrowser);
console.log(this.platform.SAFARI);
console.log(this.platform.TRIDENT);
console.log(this.platform.WEBKIT);
```

Reproduce the report with @angular/cdk 9.2.4 by logging the listed this.platform properties in a browser environment. Start by locating the CDK Platform implementation and its platform-detection tests, then compare the reported ANDROID value with the other detection flags across relevant browsers. Done means ANDROID is not true for every platform and the regression is covered by a test.

Written by the indexing model from the issue text.