Allow system_server to update timerslack_ns for hal_audio_default

Based on the following audit message:

type=1400 audit(1521738979.005:385): avc: denied { write } for pid=1269
comm="Binder:1269_B" name="timerslack_ns" dev="proc" ino=254190
scontext=u:r:system_server:s0 tcontext=u:r:hal_audio_default:s0
tclass=file permissive=1

Bug: 74110604
Test: adb shell dmesg | grep hal_audio_default
Change-Id: I4c2e787588eb9d223d5e50e1bc8f67876de97c2e