Checkbox Disabled Does Not Show Visually Checked Value

### Issue type

**I'm submitting a ...** (check one with "x")

* [] bug report
* [X] feature request

### Issue description

**Current behavior:**
The checkbox when disabled does not display the visually checked value.

**Expected behavior:**
Learn checkbox checked but disabled.

**Steps to reproduce:**
In the showcase itself in the Disabled Checkbox part.
https://akveo.github.io/nebular/docs/components/checkbox/examples#nbcheckboxcomponent

### Other information:

**npm, node, OS, Browser**
```
Node v10.16.0, Npm v6.9.0
OS: Windows 10
Browser: Chrome Versão 76.0.3809.132 (Versão oficial) 64 bits
```
**Angular, Nebular**
```
"@angular/animations": "^8.0.0",
"@angular/cdk": "^8.0.0",
"@angular/common": "^8.0.0",
"@angular/compiler": "^8.0.0",
"@angular/core": "^8.0.0",
"@angular/forms": "^8.0.0",
"@angular/material": "^8.1.2",
"@angular/platform-browser": "^8.0.0",
"@angular/platform-browser-dynamic": "^8.0.0",
"@angular/router": "^8.0.0",
"@nebular/auth": "4.1.2",
"@nebular/eva-icons": "^4.3.1",
"@nebular/security": "4.1.2",
"@nebular/theme": "^4.3.1",
```