Accessibility: Cause screen readers to read "message sent" flash message

Fixes: CNVS-12806

Screen readers were not working with the flashMessage
method. Determined that adding a timeout on it will
make the screenReaderFlashMessage method fire as
intended. This results in screen readers properly
picking up on the alert.

Test plan:
Using a screenreader
1.) Navigate to conversations
2.) Send a message
3.) Verify that the screen reader reads the "message sent" flash message


Change-Id: I83985690fc672ceb146ceed3a17b0f997d360f75
Reviewed-on: https://gerrit.instructure.com/35858
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Aaron Cannon <acannon@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>