Fix the shorcuts deletion issue in ShortcutService

Since it cleans up directories and rewrite the ShortcutPackageItems
during saveToXml(), it could lead to shortcuts missing when shutdown.

This CL receives ACTION_SHUTDOWN so that it can finish up saving it
before shutdown if the task is scheduled.

Test: compile & verify basic functions working
Bug: 178191974
Change-Id: If7dece01d6ca5280e6e596317292273246a816e4