Fall back to default custom shortcuts if custom shortcuts are disabled

Bug: b/281528589
Test: manual - customize shortcuts, disable customization, then observe falling back to defaults
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:8096070c173b6dba6453f9467fc63edcc5c5dcd4)
Change-Id: I6d74cf9960a09b7d3261154e711f824058b0d25a