Use appropriate notification channel for file transfer notifications
Summary
Previously the "Sent file to [device]" notifications would be sent to the "Other notifications" channel, which by default makes a noise/vibration. Instead, they're now sent to the "File transfer" channel, which doesn't.
(NB: I've not tested this! If you're happy to merge it regardless, go ahead, or I will work out how to build the Android app and check I've not broken anything as soon as I can.)
Test Plan
Before:
Send a file from the phone, and check that the notification was sent to the "Other notifications" channel (e.g. on Android 10, long-press the notification).
After:
Send/receive a file, and check that the notification was sent to the "File transfer" channel. It would be good to also test transferring multiple files, or file transfers which fail.