Fix flaky idmap test

The entry pool is encoded in UTF-8 not UTF-16. This would cause stringAt
to return a pointer to a cache that holds the UTF-8 string converted to
UTF-16. If the cache becomes invalidated, the UTF-16 pointer is no
longer valid and causes seg faults when looked up.

Test: libandroidfw_tests
Change-Id: I0f13dbeadd0d148de2805efd6cadb8e220368e3b