[Extensions] Add GetInstalledExtension() method to ExtensionRegistry

This CL adds GetInstalledExtension() method to ExtensionRegistry and
uses it instead of deprecated ExtensionService::GetInstalledExtension()
in chrome/browser/ui/app_list/.

Part of removing the deprecated GetInstalledExtension() call
from the ExtensionService.

BUG=489687

Review URL: https://codereview.chromium.org/1130353010

Cr-Commit-Position: refs/heads/master@{#333036}
AppListControllerDelegateImpl::AppListControllerDelegateImpl(
    AppListService* service)
    : service_(service) {}

AppListControllerDelegateImpl::AppListControllerDelegateImpl(
    AppListService* service)
    : service_(service) {}
