Shortcut: Improve backup & restore

Shortcut: Improve backup & restore

* Catch RuntimeException from restore, in case restoring from a
partner device with an incompatible file format.

* When a restore target app is already installed, and
- if it has allowBackup=true, we'll restore normally, so all
existing shortcuts will be replaced. (but manifest shortcuts
will be re-published anyway.) We log a warning on logcat.

- if it has allowBackup=false, we don't touch any of the
existing shortcuts.

Bug 31057974
Bug 30766177

Change-Id: Ic3f7e860e7ea0d086fc589d8cbed8c4cebdd4bc6