Logs show that only dumpstate requires access.

avc: granted { read open } for comm="screencap" path="/dev/ion"
dev="tmpfs" ino=14324 scontext=u:r:dumpstate:s0
tcontext=u:object_r:ion_device:s0 tclass=chr_file
avc: granted { ioctl } for comm="screencap" path="/dev/ion" dev="tmpfs"
ino=14324 ioctlcmd=4906 scontext=u:r:dumpstate:s0
tcontext=u:object_r:ion_device:s0 tclass=chr_file

Grant ion permission to dumpstate which uses it for screencap
feature.

Bug: 28760354
Test: build. Check logs.
Change-Id: I6435b7dbf7656669dac5dcfb205cf0aeda93991b