Newly create idmap only when it is stale

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only a group id will be updated when the idmap isn't outdated.

To correctly check staleness of idmap, "--verify" function of idmap is
invented as well.

Test: building succeeded and tested on sailfish.
Bug: 37179531
Change-Id: I874be9765d37bfa6c562c3f39a395040dc6a7d1f