[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 AppListControllerDelegate::UninstallApp(Profile* profile,
                                             const std::string& app_id) {
  ExtensionUninstaller* uninstaller =
      new ExtensionUninstaller(profile, app_id, this);
  uninstaller->Run();
}

void AppListControllerDelegate::UninstallApp(Profile* profile,
                                             const std::string& app_id) {
  ExtensionUninstaller* uninstaller =
      new ExtensionUninstaller(profile, app_id, this);
  uninstaller->Run();
}
