Prepare pass_through mounts

Execute mkdir -p /mnt/pass_through/0/emulated
and bootstrap the emulated volume for user 0.

The 'pass_through' paths allow us bind mount the lower
filesystem directly into /storage, bypassing any sdcardfs
of FUSE mounts.

This change is part of enabling upcoming platform changes that are
described in the bug linked below.

Bug: 135341433
Test: builds, boots and pass through directories are created
Change-Id: I46ce207d06a1ec550b8bacac259387371fc0b841