libandroidfw: Do not clear last resource id in ResolveReference

If the value passed to AssetManager::ResolveReference is not a
reference, the caller may be expecting for the last reference to
not be cleared, as a more appropriate value should most likely be
retained.

This was causing an issue when a caller was manually resolving
references and expecting the last resource ID resolved to be propagated
across calls to ResolveReference.

Test: make libandroidfw_tests
Change-Id: I5b7f586e2cd541059023eaa9ba23e324a21a9a1e