Unify the usage of "readonly" modifier for constructor parameters

[InnaAtanasova]

Currently we have both cases:

private readonly _elRef: ElementRef,
private readonly _cdr: ChangeDetectorRef,
@Optional() private readonly _rtlService: RtlService

and

private  _elRef: ElementRef,
private  _cdr: ChangeDetectorRef,
@Optional() private  _rtlService: RtlService

We need to:

• agree on the unified approach
• change all occurrences in the project - Core and Platform