Adds content descriptions for in-call buttons

[clado]

First time contributor checklist

• I have read how to contribute to this project
• I have signed the Contributor License Agreement

Contributor checklist

• I am following the Code Style Guidelines
• I have tested my contribution on these devices:

• Google Pixel 2, Android 11 (LineageOS 18.1)

• My contribution is fully baked and ready to be merged as is
• I ensure that all the open issues my contribution fixes are mentioned in the commit message of my first commit using the Fixes #1234 syntax

---

Description

Fixes #9774

It seems like #9794 would have also fixed #9774, however it seems like #9794 has been closed for some time and at this point would require resolving some merge conflicts in order to ship. Normally I would have asked whether I could help improve that PR instead of making a new one, however I didn't realize that #9774 had already been addressed until after debugging the issue and writing the code. 😅 Hopefully my PR is ok/helpful! 🙏🏻

Previously, when in a WebRTC call and using Talkback, the dailer buttons would read, from right to left:

• unlabelled button
• (on/off) switch
• (on/off) switch
• unlabelled button (when showing flip)

now, when in a WebRTC call and using Talkback, with video and audio enabled, the dailer buttons will read (from right to left):

• end call button
• on unmute switch
• on camera switch
• flip button

when in a WebRTC call and using Talkback with video off and audio muted, the dailer buttons will read (from right to left):

• end call button
• off unmute switch
• off camera switch
• flip button

I had to create a new "unmute" string in order to accommodate the way that the button currently works, but hopefully this will be a quick one to translate because the string already exists elsewhere in Signal. :) I think that the double negatives could be a bit tricky and might recommend swapping how the button announces itself, but that seemed like outside the scope of this PR.

I wasn't able to test the speakerphone button in Talkback/it seems to appear somewhat non-deterministically on my phone, but I have a high confidence that that button will work out well? 😅 🙏🏻

I do think that it would likely be a better user experience if the "flip" button read "flip to front camera" or "flip to back camera" but it looks like getting the camera state into WebRtcCallView may be fairly complicated and above my skills at this time. As it seems like this issue has been open for a long time, maybe it's helpful to merge the initial PR, then have someone else follow up later?

[clado]

It looks like this has been merged! 🎉 That was really fast, thank you!!
This PR seems like it's been resolved/made redundant, so I'm going to close it. Please let me know if this is inappropriate and we should be able to reopen it. :)