Rework SendKeyEvent in event_rewriter_unittest.

- SendKeyEvent now takes TestKeyEvent, and returns the rewritten
  key events. It will abort, if the rewritten events are not key events,
  which is currently satisfied.
- Introduced SendKeyEvents(), which is similar to SendKeyEvent but
  takes a sequence of TestKeyEvent instances.
- Removed SendActivateStickyKeyPattern. It is just a press/release
  key event pairs, so replaced by SendKeyEvents().

BUG=1440147
TEST=Tryjob

Change-Id: I8f6836f98db746cf421882ae4323bbaf08d55f51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5151647
Reviewed-by: David Padlipsky <dpad@google.com>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1243781}