[CrOS MultiDevice]: Subpage item for Android Messages

This CL Sets up the newly migrated pref to enable the Android Messages
feature to be toggled by a feature toggle in the settings subpage by a
newly added feature item. In addition to the toggle, the feature item
has an icon and a button to start the verification flow for the feature
once that is built. For the moment, the button just turns on the pref
(which is not used elsewhere so there's no security issue) and then is
replaced by a toggle.

Change-Id: I23108a2cfc540a6600c89605c953ada31cfeb914
Bug: 824568
Reviewed-on: https://chromium-review.googlesource.com/1145890
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Jordy Greenblatt <jordynass@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579648}
7 files changed