Improvements for camera privacy switch UI

When the camera privacy switch state changes, and a toast describing the
new state is shown, cancel the toast for the previous state so it gets
hidden immediately (the new toast is enqueued, so it won't hide the old
one).

Also, do not show toast when the privacy switch state changes from
unknown to off state, so "Camera is on" toast is not shown for the
default initial state.

BUG=1165937

Change-Id: Ief3be58a89b52e17a597475baa1230dc3f2fbd4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626187


Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842769}