allow system_server to set bootanim scheduling priority

Addresses the following denial:
     avc: denied { setsched } for pid=1405 comm="Binder:1094_3" scontext=u:r:system_server:s0 tcontext=u:r:bootanim:s0 tclass=process permissive=0

Maybe fix bug 30118894.

Bug: 30118894
Change-Id: I29be26c68094c253778edc8e4fef2ef1a238ee2e