Tree grid does not successfully indent after datasource being re-instantiated

### Issue type

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

* [X] bug report
* [ ] feature request

### Issue description

**Current behavior:**

When re-instantiating `this.dataSource`, the TreeGrid fails to indent any entries that have been programmatically expanded (using the `expanded` field or `this.dataSource.expand` or `this.dataSource.toggle`) but does indent entries when clicking them.

**Expected behavior:**
Elements should indent properly, when the `this.dataSource` is re-instantiated

**Steps to reproduce:**
Code is same as from showcase from this [page](https://akveo.github.io/nebular/docs/components/tree-grid/examples#nbtreegridcomponent).
After having set the DataSource using the DataSourceBuilder
```ts
this.dataSource = this.dataSourceBuilder.create(this.data);
```
i make delete an entry from `this.data` and to reflect this, i re-instantiate `this.dataSource`
```ts
this.data.forEach(m => {
m.children.forEach(sa => {
const index = sa.children.findIndex(q => q.data.id === deleteId);
if (index !== -1) {
sa.children.splice(index, 1);
}
});
});
this.dataSource = this.dataSourceBuilder.create(this.data);
```
Now it will reset the `expanded` statuses to what was set in `this.data`. However it does not indent those expansions. It also won't indent anything that is expanded using `this.dataSource.expand` or `this.dataSource.toggle`.
When expanding an entry by clicking on it, it however successfully indents all the entries, that have not been indented properly.

### Other information:

**npm, node, OS, Browser**
```
Node: 10.13.0
NPM: 6.4.1
OS: Ubuntu 18.04
Browser: Chrome
```

**Angular, Nebular**
```
{
"name": "web",
"version": "0.0.0",
"scripts": {
"ng": "ng",
"start": "ng serve",
"build": "ng build",
"test": "ng test",
"lint": "ng lint",
"e2e": "ng e2e"
},
"private": true,
"dependencies": {
"@angular/animations": "~8.2.0",
"@angular/cdk": "^8.0.0",
"@angular/common": "~8.2.0",
"@angular/compiler": "~8.2.0",
"@angular/core": "~8.2.0",
"@angular/forms": "~8.2.0",
"@angular/platform-browser": "~8.2.0",
"@angular/platform-browser-dynamic": "~8.2.0",
"@angular/router": "~8.2.0",
"@nebular/auth": "^4.1.3",
"@nebular/eva-icons": "4.1.3",
"@nebular/moment": "^4.1.3",
"@nebular/security": "^4.1.3",
"@nebular/theme": "^4.1.3",
"eva-icons": "^1.1.1",
"rxjs": "~6.4.0",
"tslib": "^1.10.0",
"zone.js": "~0.9.1"
},
"devDependencies": {
"@angular-devkit/build-angular": "~0.802.2",
"@angular/cli": "~8.2.2",
"@angular/compiler-cli": "~8.2.0",
"@angular/language-service": "~8.2.0",
"@schematics/angular": "~8.2.0",
"@types/jasmine": "~3.3.8",
"@types/jasminewd2": "~2.0.3",
"@types/node": "~8.9.4",
"codelyzer": "^5.0.0",
"jasmine-core": "~3.4.0",
"jasmine-spec-reporter": "~4.2.1",
"karma": "~4.1.0",
"karma-chrome-launcher": "~2.2.0",
"karma-coverage-istanbul-reporter": "~2.0.1",
"karma-jasmine": "~2.0.1",
"karma-jasmine-html-reporter": "^1.4.0",
"protractor": "~5.4.0",
"ts-node": "~7.0.0",
"tslint": "~5.15.0",
"typescript": "~3.5.3"
}
}
```