Avoid audit when running `adb shell -t run-as xxx`.

run-as uses file descriptor created by adbd when running
`adb shell -t run-as xxx`. It produces audit warnings like below:

[ 2036.555371] c1    509 type=1400 audit(1497910817.864:238): avc: granted { use } for pid=4945 comm="run-as" path="/dev/pts/0" dev="devpts" ino=3 scontext=u:r:runas:s0 tcontext=u:r:adbd:s0 tclass=fd

Bug: http://b/62358246
Test: test manually that the warning disappears.
Change-Id: I19023ac876e03ce2afe18982fe753b07e4c876bb