Refactor Trusted Web Activity UI code to MVC

The code is restructured according to the high-level MVC picture, but
low-level details such as view binding mechanism are not used, as
it seems to be an overkill at this point. We can easily install those
mechanisms in future should the model become more complicated.

Verification is extracted to TrustedWebActivityVerifier in the
controller layer. Several controllers observe verification and update
the model. Several independent Views are observing the changes in the
model and updating themselves accordingly.

Change-Id: I8882da2d648fcea21b85fd4a1c0568dd00e47113
Reviewed-on: https://chromium-review.googlesource.com/c/1323069
Reviewed-by: Peter Conn <peconn@chromium.org>
Commit-Queue: Pavel Shmakov <pshmakov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606853}
21 files changed