Add interface to event router so it can be mocked in a test

Need this to mock this class in a test case coming
for gallium bridge.

Also missing dispatch method.

Bug: None
Test: None
Change-Id: I79d5e0408ccd58779d307cf38944b3d35f885e32
Reviewed-on: https://chromium-review.googlesource.com/c/1417717
Reviewed-by: Sergey Volk <servolk@chromium.org>
Commit-Queue: Randy Rossi <rmrossi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623794}
4 files changed