Newly create idmap only when it is outdated

For now, OverlayManagerService calls Installd.idmap() whenever a user
is changed, and then a idmap is re-generated even though there are no
changes on its target apk and overlay apk.
This CL is to avoid such unnecessary re-generation of idmap. Instead
only a group id will be updated when the idmap isn't outdated.

Test: building succeeded and tested on sailfish.
Bug: 37179531
Change-Id: Id726cbbc621bd64f2a329798b2ab5436f0c28d70