Allow system_server to measure emulated stats.

Historically we pushed all system_server SD card interactions through
DefaultContainerService to avoid holding open FDs, but it's safe to
measure disk usage for internal emulated storage when looking
directly at /data/media, since there is no risk of unsafe ejection.

Test: builds, boots, and access allowed
Bug: 33298975
Change-Id: I1491065aad47cd16e7fdd586bce021ff61c003c6