Fixup SDCARD_RW GID for multi-user.

sdcardfs takes the user-id into account when synthesizing the GID for
package-specific data/obb directories; so make sure we grant the GID
based on user-id as well.

Bug: 148094468
Test: OBB installs as secondary users works
Change-Id: Ibd73d8f2ef7dda14605a6095671a587c2316fc90