idmap2: Set aidl_return to true on success

idmap2d did not set the aidl return value to true when
IdmapHeader::IsUpToDate succeeded. This regressed OMS initialization
time.

This change sets aidl_return to true when the header is up-to-date and
also replaces usages of `uint32_t` for fulfilled_policies with
`PolicyBitmask`.

Bug: 158491243
Bug: 158535227
Bug: 158468195
Test: enable/disable overlays and onserve that verify succeeds
Test: change the values of enforce overlayable and fulfilled policies
      and confirm that idmap verify fails
Change-Id: If8febbfd0283d53211263690f85e2970706a33b7