Image thumbnail of video leaks potentially sensitive data

Bug description

When a user creates a video, they can trim it before sending it to one of their contacts. Sadly, the thumbnail is generated before the image is cropped, thus it can contain information that the user intentionally omitted.

Steps to reproduce

• Create a video, preferably with moving objects
• Crop the video
• See how the thumbnail contains information, not available in the cropped version

Screenshots

This is the image that the user sees

Screenshot_20230419-153734_Signal

And these are the two video's that the user sent

• https://user-images.githubusercontent.com/33983090/233094877-b0a7b20e-028f-4d2c-84da-66bc03980358.mp4
• https://user-images.githubusercontent.com/33983090/233096337-be157948-ea4f-43a4-958d-f6fea1249924.mp4

Note how one video is cropped and one isn't, while the thumbnail is all the same.

Device info

Device: One Plus 7 Pro
Android version: Lineage OS 20
Signal version: 6.17.3

Link to debug log

https://debuglogs.org/android/6.17.3/4d759a5aab1e1f954350feb5840837eeb34e2a09fc69699a36a8bf7f89399971

Start by reproducing the issue with a video that is cropped before sending, using the reproduction steps and linked media as reference. Trace the video-cropping and thumbnail-generation entry points, then verify that the thumbnail reflects only the cropped video and does not reveal omitted content.