Allow multiple emulated storages at the same time.

Only do multiuser mapping for primary emulated storage as per google docs.
Allow all non-primary emulated storage to have id other than 0 so that
it could be distinguished from the primary volume.

Change-Id: I7b70b9067fd6211055fb9074710f48107d7b0e8e
Revert "Allow multiple emulated storages at the same time."

This reverts commit a83ee097b5c35cefaec2c4b7a8e313816634fd0f.