(iOS) Added preferences for privacy on background only and overriding LaunchImage

[MobilityTiago]

Description

In iOS activating privacy in WillResign method generates a behavior that isn't desirable in current project I'm working on (i.e. Shows privacy screen in Touch ID prompt), as such a preference "PrivacyOnBackground" is now checked, if it is true then privacy screen is only activated on background. Another preference "PrivacyOverrideLaunchImage" has been added so that even if a launch image is set, if this preference is true it overrides to the "Default" image.

Related Issue

https://github.com/devgeeks/PrivacyScreenPlugin/issues/33

Motivation and Context

On our app that uses this plugin, when the Touch Id Prompt was launched the privacy screen blocked the app view. The app has a launch image, but it has been defined in our app that the privacy screen must be different

How Has This Been Tested?

Manual testing to check if intended behavior was added, manual testing to check if previous behavior was kept without changes to current users of the plugin.

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [X ] My code follows the code style of this project.
• [X ] My change requires a change to the documentation.
• [X ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[devgeeks]

I would have liked the fix for the Touch ID issue, but wish it hadn't been wrapped up with another feature. Maybe you could find a way to make this two different PRs?