Return error in "sm unmount-app-data-dirs" if app data isolation is not enabled

This command should not do anything when feature flag is off,
and let developer knows this command fails.

Bug: 177228735
Test: run "sm unmount-app-data-dirs" when the app data isolation flag is off and returns error message
Change-Id: If07107712e8215a58ffade08aedce3e80ad1ac12