system_server: Allow hwservicemanager to make binder calls

Fixes the following denial:
avc: denied { call } for pid=791 comm="system_server" scontext=u:r:system_server:s0 tcontext=u:r:hwservicemanager:s0 tclass=binder permissive=1

Test: Builds, boots, vibrator works on bullhead
Change-Id: I56a0a86b64f5d46dc490f6f3255009c40e6e3f8f