Add policies and enforce overlayable to header

If the fulfilled policies change without the contents of the target
and overlay APKs changing, the idmap for the overlay should be
regenerated. This change adds fulfilled policies and enforce
overlayable to the idmap header so that idmap2d can determine if the
polices or enforce overlayable changed from what was used to generate
the idmap.

Bug: 119328308
Test: idmap2_tests
Test: atest RegenerateIdmapTest

Change-Id: I96f970e82b5243be01b205ac2cb6ab249c6100bc