Always delete old idmap in createIdmap

If the target package updates its policies, and the overlay fails to
successfully overlay any resources, the overlay should no longer work
anymore. By deleting the old idmap file at the beginning of
Idmap2Service::createIdmap, the overlay state will change to
STATE_NO_IDMAP if the new idmap cannot be successfully generated.

Bug: 173032050
Test: manual
Change-Id: I1915e65feeb801d8602c12cc7df0f90da4c9b5c5