Forces OveriewInputConsumer to be used when Assistant is running on top of Launcher.

Previously it was using OtherActivityInputConsumer, which got things in a
pretty weird state (e.g. most recent app would appear in the center as if
it was the active app when you started Quick Switching, etc.).

By default (toggleable by a feature flag), OverviewWithoutFocusInputConsumer
is used because Assistant doesn't seem to respect the CLOSE_SYSTEM_DIALOGS
broadcast, at least in half-shelf mode. In this case, the Home intent is
sent on swipe up, or you can dismiss it with the back gesture or by tapping
above the half shelf.

The new feature flag ASSISTANT_GIVES_LAUNCHER_FOCUS routes touches through
OverviewInputConsumer. As opposed to OverviewWithoutFocusInputConsumer,
this allows Quick Step to work while Assistant is running. Additional logic
is added to dismiss the Assistant when appropriate. Note that the dismissal
happens atomically, so it's not completely fluid with the other animations.
As mentioned above, this is disabled by default because Assistant doesn't
currently respect CLOSE_SYSTEM_DIALOGS.

Demo with the flag enabled (and Assistant respecting CLOSE_SYSTEM_DIALOGS):
https://drive.google.com/open?id=1W5jGpn_TEC-KjrYwQtaBT3pzxG_5tC4W

Bug: 139661510
Change-Id: I261653118aff289b329ec2a7ca6e52f100f7835a
Tested: Manually used quick switch after invoking Assistant from home screen.