Pass correct install user when creating sessions

This CL passes the correct install user when creating sessions in
StagingManager and effectively reverts ag/6901350.

Add a test to ensure b/129397974 is not regressed as described in
b/158222747#comment11.

Fix: 129744602
Fix: 158222747
Test: atest MultiUserRollbackTest#testStagedRollback
Change-Id: I718992240aa76898ff9e4220ea6a769ee8cd61f5