init: use libfscrypt for file-based encryption setup

The file-based encryption setup code is being refactored into its own
library because it applies to both ext4 and f2fs, not just ext4.  Update
init to use the new location.   For fs_mgr, just remove the include of
ext4_crypt_init_extensions.h since it was unneeded.

Test: built, booted device with f2fs encryption
Change-Id: I392a763e3349f001bdbc09eb9ca975aa3451fd68