[Autofill Assistant] Fix transition from overlay to TouchEventFilter.

https://chromium-review.googlesource.com/c/1331401 introduced a bug that
made the page visible temporarily between the time the overlay is
switched off and the time a touchable element appears.

This was due to an over-eager early abort in updateTouchableArea, which
would not update anything unless the touchable area changed - regardless
of whether enabled changed.

This patch fixes the problem by decoupling the update of "enabled" from
the update of the touchable area.

Bug: 806868
Change-Id: I3ee78489cb538d6fd06e24d15fcb47a456b1f079
Reviewed-on: https://chromium-review.googlesource.com/c/1333754
Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607630}