idmap2: lock down write access to /data/resouce-cache

Deny write access to /data/resource-cache for UIDs other than root and
system. While this is already handled by SELinux rules, add an
additional layer of security to explicitly prevent malicious apps from
messing with the system's idmap files.

Test: make idmap2_tests
Change-Id: Id986633558d5d02452276f05f64337a8700f148a