Allow switching between sdcardfs and FUSE mounts

This change is part of enabling upcoming platform changes that are
described in the bug linked below.

Bug: 135341433
Test: builds, boots successfully and external storage remains
an sdcardfs mount by default and works correctly
Test: cat /proc/1/mountinfo is unchanged

Change-Id: Idf851b3a42910e0ce8fdd75daea1cce91dd1aa98