Remove std::move from observer_ of mojo::Remote in EcheApp signaler

We don't pass ownership of the observer_ member so doing std::move()
isn't necessary.

Bug: b/187761187
Change-Id: I54b32679d318d41410bd79516d747fca956c43a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2885399


Commit-Queue: Guanru Lee <guanrulee@chromium.org>
Reviewed-by: Daniel Nishi <dhnishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#882423}