Maintain compatibility by deleting target directory before renaming into it.

The target shouldn't be a directory, but if it is, it would be
deleted (as long as it's empty). This became some kind of API and we
need to remain compatible with it.

Bug: 151959443
Test: Reboot and ensure ShortcutService can persist its state
Change-Id: I11a80cd4252128b025912b7aab86b113935e549a
Merged-In: I11a80cd4252128b025912b7aab86b113935e549a