[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}
void ExtensionAppItem::Launch(int event_flags) {
  const Extension* extension = GetExtension();
  if (!extension)
    return;

  if (!extensions::util::IsAppLaunchable(extension_id_, profile_))
    return;

  if (RunExtensionEnableFlow())
    return;

  GetController()->LaunchApp(profile_,
                             extension,
                             AppListControllerDelegate::LAUNCH_FROM_APP_LIST,
                             event_flags);
}

void ExtensionAppItem::Launch(int event_flags) {
  const Extension* extension = GetExtension();
  if (!extension)
    return;

  if (!extensions::util::IsAppLaunchable(extension_id_, profile_))
    return;

  if (RunExtensionEnableFlow())
    return;

  GetController()->LaunchApp(profile_,
                             extension,
                             AppListControllerDelegate::LAUNCH_FROM_APP_LIST,
                             event_flags);
}
