Sessions committed later shouldn't cause earlier ones to fail

When 2 sessions have overlapping packages, the one committed earlier
should pass the overlapping check while the later one should fail.

1. Add a field to denote the time when a session is committed.
2. When doing overlapping check, only consider sessions that are
   committed before the current session.

Bug: 174442862
Test: atest StagingManagerTest PackageInstallerSessionTest

Change-Id: Ieeafc879f32b1228d451307b194d65146c7496c1