Convert MissingImageHandler to static

[david-allison]

Purpose / Description

We warned a user once per deck about missing images, this was annoying to them as the missing images were intentional.

We only want to warn the user once per application session

Fixes

Fixes #7174

Approach

Make the variable static - enforced to be the application lifetime.

How Has This Been Tested?

Tested on my Android 9

Learning

We're creeping closer to a time where DI would have benefits.

Checklist

• You have not changed whitespace unnecessarily (it makes diffs hard to read)
• You have a descriptive commit message with a short title (first line, max 50 chars).
• Your code follows the style of the project (e.g. never omit braces in if statements)
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code

[mikehardy]

LGTM