fix: multimedia options opening more than once

[criticalAY]

Purpose / Description

Root Cause of the issue was:

• Persistent Flow Subscription: Each time handleMultimediaActions is called, a new subscription to the multimediaAction flow is created, leading to multiple active observers.
• Race Condition: When an action is emitted, all active subscriptions are notified, causing the action to be handled multiple times.

Fixes

• Fixes BUG: Multimedia Option Open Twice When Bottom Sheet is Opened Twice #17453

Approach

Keep track of the job

How Has This Been Tested?

Tested on Google emulator API35

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

[lukstbit]

Ok, this seems to fix the linked issue at least

Note: there's a bug if MultimediaImageFragments's activity gets killed during the user choosing an image for example. If this happens the user will be stuck in the choosing screen.

How to reproduce: enable "Don't keep activities" -> NoteEditor -> Attach Icon -> go and select "Open gallery" -> try to select an image -> stuck in the image chooser screen

It would be nice to fix this issue in this PR(it's the same opening multiple times but for a different reason) but I would also accept merging this and creating an issue with the bug above.