Return to modifying raw /mnt/media_rw paths.

We thought we could push everyone through sdcardfs, but secondary
devices mounted in a stable location don't give full write access to
apps holding WRITE_EXTERNAL_STORAGE, so system internals still need
to reach behind sdcardfs.

To keep sdcardfs in the loop about changes that we make behind its
back, we issue access(2) calls which should be enough for it to
invalidate any cached details.

Bug: 74132243
Test: manual
Change-Id: I727cd179a5a825b16ec4df6e2f41a079758d41c5