Remove runBlocking calls in AbstractFlashcardViewer and BrowserColumnSelectionFragment

[nikosavola]

Purpose / Description

Fixes two TODOs about runBlocking calls and refactors to the existing launchCatchingTask helper.

Approach

Remove fully blocking calls with the launchCatchingTask helper

How Has This Been Tested?

Ran ./gradlew :AnkiDroid:testPlayDebugUnitTest

Checklist

Please, go through these checks before submitting the PR.

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[criticalAY]

LGTM, thanks!