MvxAndroidViewsContainer: Remove NewTask flag as default

[nmilcoff]

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

Code improvement

⤵️ What is the current behavior?

Each time a new Activity is launched on Android, by default the flag NewTask is added.

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

The behaviour described above is fixed.

As @davidschwegler commented on the original issue:

NewTask is generally only used in rare cases where the calling app wants to switch to a completely different activity task (eg a different app's activity stack), rather than placing the called activity on top of your own stack.

💥 Does this PR introduce a breaking change?

Probably no, but we need to communicate this in the release blog.

🐛 Recommendations for testing

/

📝 Links to relevant issues/docs

Closes #962

🤔 Checklist before submitting

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