Fixup app-private dirs that are not writable.

Fixup non-writable app-private dirs that could have been left around by
an earlier race condition in the setup of the FUSE filesystem.

Bug: 181726193
Bug: 182088154
Test: erase userdata, check Youtube dirs are fixed up correctly
Change-Id: I7972f79d14893571289a2245f98e90e6c8c69f0a