The perfetto binary (the frontend to traced) reads an input config
from stdin. This CL adds allows perfetto to read the config
from adb shell when the user is rooted

Sample denials:
avc: denied { read } for comm="perfetto" path="pipe:[92340]"
dev="pipefs" ino=92340 scontext=u:r:perfetto:s0 tcontext=u:r:su:s0
tclass=fifo_file permissive=0
avc: denied { read } for comm="perfetto" path="pipe:[92491]"
dev="pipefs" ino=92491 scontext=u:r:perfetto:s0 tcontext=u:r:su:s0
tclass=fifo_file permissive=0

Test: adb root
adb shell
echo 'duration_ms: 1000;' > /sdcard/config
cat /sdcard/config | perfetto --txt -c - -d

Change-Id: I12042dfa9a2c262cec907f0231ce2184f46d1be8