VolumeManager: limit the scope of remountUid post fork.

VolumeManager: limit the scope of remountUid post fork.

We want to be sure we're not allocating memory, holding locks
or otherwise preventing the child process from making progress.

This is a temporary fix of limited scope. In the medium term, it
would be preferable to exec a binary that performs this work for us
as soon as we fork.

Test: manual
Bug: 141678467

Change-Id: I57dbd9b3c887aa27e2dd609abf0ad43c66f4ef2a