AppListController refactoring part 1: AppListShower implementation.

This is the first part of ash::AppListController refactoring preparing it
for the Mus+ash use.
The goal of the refactoring is to break up ash::AppListController into two
parts: AppListShower (implemented in ui/app_list/) and AppListDelegate
(implemented in ash/wm). In Mus+ash world, app list will live in the chrome
process, and mash/SysUI will interact with it via a thin interface.
More details in the design doc:
https://docs.google.com/document/d/1M9vqTTuprMssRXs8UIVaFjQUq3lUxSzwrWoPjPvca6Q/edit?ts=56d0e9f0#heading=h.b2znwtuxd2x2

This CL adds interfaces for AppListShower and AppListDelegate, as well as the
default implementation of the shower: AppListShowerImpl and unit tests.

BUG=557408

Review URL: https://codereview.chromium.org/1830293002

Cr-Original-Commit-Position: refs/heads/master@{#385526}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 216ff0ba63afa3ba3ec066d12fac41df95fdc7e0

all.gyp

@@ -469,6 +469,7 @@
           'dependencies': [
             '../ash/ash.gyp:ash_unittests',
             '../ui/app_list/app_list.gyp:app_list_unittests',
+            '../ui/app_list/shower/app_list_shower.gyp:app_list_shower_unittests',
             '../ui/aura/aura.gyp:aura_unittests',
             '../ui/compositor/compositor.gyp:compositor_unittests',
           ],