Only restorecon CE storage after unlocked.

On FBE devices, the filenames inside credential-encrypted directories
are mangled until the key is installed.  This means the initial
restorecon at boot needs to skip these directories until the keys
are installed.

This CL changes the implementation of the "restorecon_recursive"
built-in command to use the new SKIPCE flag to avoid labeling files
in CE directories.  vold will request a restorecon when the keys
are actually installed.

Bug: 30126557
Change-Id: I320584574a4d712c493b5bbd8a79b56c0c04aa58