Removed code smells by external developer

[alperenselcuk]

Task/Issue URL: Tech Design URL: CC:

Description: Hi, this is my first contribution to open source, please let me if I'm wrong. thnx you!

AppIconManager should be final class because that doesnt conform any class

AppIconManager's init should be private because this class used as singleton

Result type of changeAppIcon function was rewrite as more swifty

Removed some white spaces in FaviconsHelper.swift

Steps to test this PR:

1. Go to settings > Appearance > App Icon
2. Change icon
3. Observe app icon

Definition of Done (Internal Only):

• [ ] Does this PR satisfy our Definition of Done?

Copy Testing:

• [ ] Use of correct apostrophes in new copy, ie ’ rather than '

Orientation Testing:

• [ ] Portrait
• [ ] Landscape

Device Testing:

• [ ] iPhone SE (1st Gen)
• [ ] iPhone 8
• [ ] iPhone X
• [ ] iPhone 14 Pro
• [ ] iPad

OS Testing:

• [ ] iOS 14
• [ ] iOS 15
• [ ] iOS 16

Theme Testing:

• [ ] Light theme
• [ ] Dark theme

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Internal references:

Software Engineering Expectations Technical Design Template

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