RollbackManager: fix installedUsers null pointer exception.

The installedUsers argument is passed as null to
enableRollbackForSession in the pre-reboot case of staged installs, but
the code assumes it is non-null. Pass an empty array instead and ensure
the installedUsers is properly updated post-reboot.

Bug: 124284714
Test: atest StagedRollbackTest (currently under development)
Change-Id: I4ccd19ca289fd127f77c070c0ad0e2873a4e692a