Allow key downgrade during staged install of apex packages

Downgrade is permitted only for the system server initiated
sessions. This is enforced by INSTALL_ALLOW_DOWNGRADE flag
parameter.

Bug: 136002636
Test: atest RollbackManagerHostTest#testApexKeyRotationStagedRollback
Change-Id: I0c0e6eaee65a6c77f62425c3929ffa805d1e0f47
Merged-In: I0c0e6eaee65a6c77f62425c3929ffa805d1e0f47