app_list: Move AppListItemMetadata out of mojo

- Declare struct AppListItemMetadata in app_list_types.h
- Remove mojom::AppListItemMetadata
- AppListItemMetadataPtr -> std::unique_ptr<AppListItemMetadata>

Bug: 958134
Change-Id: Ia2368ccf5e657ed16f2db38d7c6a43bd6b5469a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1620571
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661522}
23 files changed