Fix messaging when screen recording stops without recording content.

If there's some sort of error when stopping a screen recording, the
toast and the notification should say "Error saving screen recording"
not "Error starting screen recording".

Bug: 356586297
Bug: 332662551
Flag: EXEMPT bugfix

Test: screen record a single app, but then go to home screen before
countdown finishes, then click on status bar chip and click "Stop" (or
click "Stop" on the notification) -> verify toast says "Error saving
screen recording" and verify notification says "Error saving screen
recording"
Test: atest RecordingServiceTest

Change-Id: I0c19ab9ee121fd7d47352653062fe8ecbe870f5c