Add syncs when creating parent directories

vold creates some directories for storing encryption keys if they don't
already exist, potentially including parent directories:

    /metadata/vold/metadata_encryption
    /data/misc/vold/volume_keys/$volume_uuid
    /data/misc_de/$user/vold/volume_keys/$volume_uuid
    /data/misc_ce/$user/vold/volume_keys/$volume_uuid

Currently fs_mkdirs() is used for this.  However, fs_mkdirs() doesn't
include the fsync()s of the parent directories that are needed to ensure
that the new directories are persisted to disk right away -- which is
important for encryption keys.

Add a utility function MkdirsSync() which does what is needed, and make
the appropriate places call it.

Test: Booted and checked log for "Created directory" message.
      Also ran 'atest vold_tests' to run the new unit test.
Change-Id: Ie9917b616433080139b8db3fd6877203ee6faf77