ime: Add metrics for mode change key.

We want to know:

1) Overall usage
2) What was result of pressing the key.

We also expanded ImeSwitch enum to include the key.

Bug: 953901
Change-Id: I0a59bfd836cdd9c56d68449f3417dfda7132ef18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619445
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661500}
4 files changed