Write original message timestamp into saved attachment file metadata.

[Trigus42]

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• I am following the Code Style Guidelines
• I have tested my contribution on these devices:
  • OnePlus Nord 4, Android 16 (OOS 16.0.5)
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Description

Fixes #14584 - saved attachments appearing with the save date instead of the original message date in the gallery.

Commit 237ac9f set DATE_ADDED and DATE_MODIFIED in the Android MediaStore database when saving attachments. The MediaStore is Android's media index - a SQLite database that catalogs files on the device. However, many gallery apps read dates from embedded file metadata rather than MediaStore columns. In my testing on a OnePlus Nord 4 (Android 16), saved attachments still appeared sorted by save date after that commit.

This PR writes the original message timestamp (dateSent) directly into the saved file:

• EXIF metadata for JPEG, PNG, and WebP images - sets DateTime and DateTimeOriginal via AndroidX ExifInterface. DateTimeOriginal is used here because Signal strips EXIF before sending, so no original capture date exists in the file - the sent date is the best available approximation and is what gallery apps sort by.
• MP4 container timestamps for MP4-family videos (video/mp4, video/3gpp, video/3gpp2, video/mp2ts, video/quicktime) - binary-patches creation_time and modification_time in the mvhd, tkhd, and mdhd boxes in-place, without loading the full file into memory.

Other formats (GIF, WebM, etc.) are skipped gracefully - the save still succeeds, just without embedded timestamp metadata.

Testing: Bulk-saved all 4,533 attachments from a real Signal account on the device above. 4,529 succeeded (4 failures were pre-existing missing attachment data, unrelated). Verified correct timestamps via ffprobe on saved videos and an EXIF reader on saved images.

Note: This code was written with AI assistance and tested by a human with real-world data on a physical device.