crash_dump_fallback: allow writing to system_server pipes.

Allow mediacodec/mediaextractor to write to system_server pipes during
ANR dumps.

Addresses the following denials:
avc: denied { write } for comm="mediaextractor" path="pipe:[1177610]" dev="pipefs" ino=1177610 scontext=u:r:mediaextractor:s0 tcontext=u:r:system_server:s0 tclass=fifo_file permissive=0
avc: denied { write } for comm="omx@1.0-service" path="pipe:[1175808]" dev="pipefs" ino=1175808 scontext=u:r:mediacodec:s0 tcontext=u:r:system_server:s0 tclass=fifo_file permissive=0

Bug: http://b/63801592
Test: treehugger
Change-Id: I944b1fa76c70402607ccd903be17dbddeaa73201
(cherry picked from commit 3c9b9197)