servicemanager: lookup service labels using "service" backend

Type 0 actually corresponds file_contexts backend. Use type
SELABEL_CTX_ANDROID_SERVICE instead.
https://android.googlesource.com/platform/external/libselinux/+/d4828b3ba449355aa2d1751026e5f1d1129cedcf/include/selinux/label.h#27

Test: boot cuttlefish; no denials to any service
Change-Id: I1c33c31ecbd46168998606c3463837a9c6f39f3a