Refactor shortcut updating, and do it on BOOT_COMPLETE

This CL shuffles quite a bit of code around, but the effective differences are:
* Unified shortcut updating code (language switch & backup restoration).
* Shortcuts are also updated on boot (flagged by MODES_UI which will need this).
* Removed usage of (long obsolete) AsyncTask.

A further CL will add some special-casing for the DND->Modes shortcut transition.

Bug: 365545604
Test: atest com.android.settings.shortcut + manual (switch language, reboot)
Flag: android.app.modes_ui
Change-Id: I30450d13cb05008d2a71ed89d4781eb81e5532b9