Fix logspam when user removed before CE storage prepared

Due to frameworks/base commit 5c65b1ee1023 ("Don't prepare CE storage on
user creation") (http://ag/20241697), removing a user immediately after
creating it causes the user's directories to be destroyed before CE
storage was prepared.

Functionally this works fine; however, it causes some error messages to
be spammed to the log because 'vold_prepare_subdirs destroy' doesn't
like that /data/misc_ce/$userId and /data/vendor_ce/$userId don't exist.
vold_prepare_subdirs logs two error messages itself, but it also exits
with a failure status, which bubbles up and causes a Slog.wtf with a
stack trace in StorageManagerService.

Fix this by making rmrf_contents() simply return true if the directory
doesn't exist.

Bug: 232452368
Test: 'pm create-user 10 && pm remove-user 10' and check logcat
Change-Id: I867a915f4b25e1a5f0603fbd84680b673ff5eb96