Android Dialogs: Fix close & do not keep references to instances

[nmilcoff]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Bug fix

⤵️ What is the current behavior?

1. When the screen is rotated, dialogs are recreated and instances held in our ViewPresenter are obsolete. Therefore when trying to close a dialog after a rotation -> 💣 💥
2. Json plugin has a useless definition of Preserve attribute

🆕 What is the new behavior (if this is a feature change)?

1. Above is fixed. We obtain the Dialog to close from the fragment manager.
2. That preserve attribute is removed.

💥 Does this PR introduce a breaking change?

No.

🐛 Recommendations for testing

1. Run Playground.Droid
2. Tap on "Show modal"
3. Rotate the screen
4. Close the dialog

On current develop this will fail, on this branch it won't.

📝 Links to relevant issues/docs

Fixes #2378

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated (docs style guide)
• Rebased onto current develop