Refactor extensions/Context.kt slightly

No behaviour change intended.

What is it?

• [ ] Bugfix
• [ ] Feature
• [x] Codebase improvement

Description of the changes in your PR

• Simplify insertOrUpdateConversation(), since the previous code was long and hard to follow.
• Inline constants into selection queries instead of using '?' arguments.

Before/After Screenshots/Screen Record

N/A

Fixes the following issue(s)

N/A

Acknowledgement

• [x] I read the contribution guidelines.