Announce download and upload percentage to screen readers

[mehranlatifi83]

Summary

Download and upload progress is only partly available to screen reader users. Documents, GIFs and videos announce the transferred size but never the percentage, and other media announce nothing at all while they are being transferred.

This change:

• Announces the transfer as "Downloading 25%, 2 MB of 34 MB" when the message is reached, so the percentage comes before the sizes that were already announced.
• Announces "Downloading 25%" for media that has no size to report, such as photos, voice messages and music, which previously stayed silent.
• Follows the transfer while the message is being read: every 10% while the finger rests on it, and at most every 2.5 seconds while it only holds accessibility focus, and only when it moved at all. Messages that are neither touched nor focused stay silent.
• Keeps those updates to what changed, the percentage and how much of the file arrived, since reaching the message already told what it is doing and how large it is.
• Removes the duplicate upload announcement, where the same message reported both its uploaded size and a separate "Sending" percentage.
• Keeps the spoken text in sync with the button state, so cancelling a download drops the progress from the message instead of leaving the last value behind.

Whether the transfer is running is taken from the same button state the visible icon uses, so audio, music and video all resolve correctly. Nothing changes visually, and only existing Telegram strings are used.