Hide constructors of utility classes

[vil02]

Describe the bug in a sentence or two.

These classes:

• BackgroundStrategyProvider,
• BitmapUtils,
• CloudinaryHelper,
• Logger,
• PayloadFactory,
• StringHelper,
• TestUtils,
• UploadWidgetResultProcessor,
• UploadWidget,
• UriUtils,
• Utils,
• Utils,

could be proper utilities (i.e. marked as final with private default constructor). Such change makes it harder to misuse them (prevents instantiation and inheritance) and makes the coverage reports more accurate.

Issue Type (Can be multiple)

[ ] Build - Can’t install or import the SDK
[ ] Performance - Performance issues
[ ] Behaviour - Functions aren’t working as expected (Such as generate URL)
[ ] Documentation - Inconsistency between the docs and behaviour
[x] Other (Specify) - code quality

Related to #173.