Fix MissingMethodException when using MvxImageView pre-Android API 21

[mvanbeusekom]

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

Bug fix

⤵️ What is the current behavior?

When using the MvxImageView control on Android API 20 and lower (everything before API 21) you'll get a MissingMethodException. This is caused due to the new constructor ImageView(Context context, AttributeSet attrs, int defStyle, int defStyleRes) which was only added in API 21.

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

The MissingMethodException is no longer thrown because the new constructor is only called for API 21 and higher.

💥 Does this PR introduce a breaking change?

No

🐛 Recommendations for testing

Create a simple Android application which targets any API lower then 21 and include a MvxImageView control.

📝 Links to relevant issues/docs

The following external references exists:

• http://blog.danlew.net/2016/07/19/a-deep-dive-into-android-view-constructors/
• https://github.com/koral--/android-gif-drawable/issues/197

🤔 Checklist before submitting

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