[TTS] Add preserve-tap arm for the longpress exper

Adds another experiment arm for preserving the Tap gesture, in case
users get confused during the transition to long-press.  This will
let us evaluate the difference between preserving and just switching.

This changes the default to ONLY recognize longpress rather than both.

Note that Privacy wants only one gesture to fully launch, so we are
doing this as a fallback in case both need to be supported during a
user-education timeframe.

BUG=956277

Change-Id: I1b42d86bbd9cdc7e0a58bafb8a796601ae0cb353
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2076338
Auto-Submit: Donn Denman <[email protected]>
Commit-Queue: Theresa  <[email protected]>
Reviewed-by: Theresa  <[email protected]>
Cr-Commit-Position: refs/heads/master@{#745051}
7 files changed