Remove write permission from file mode of top-level user dirs

https://r.android.com/2620458 is removing the write mode bit from the
top-level user directories on internal storage, in order to make the DAC
consistent with the SELinux policy.

This commit makes the corresponding change to adoptable storage.

Bug: 285239971
Test: sm set-virtual-disk true; sm partition disk:7,392 private
Change-Id: I17dfbe10909b34c2046a4d5b4ffd7764d5ae083b