Allow shell serial_device read-write access

When starting the emulator, the system console writes entries
to /dev/ttyS2. We need to allow the writes, otherwise this generates
denials when you run "emulator -verbose -logcat '*:v' -show-kernel"

Addresses the following denial:
type=1400 audit(1395076594.320:446): avc:  denied  { read write } for  pid=5600 comm="sh" path="/dev/ttyS2" dev="tmpfs" ino=1487 scontext=u:r:shell:s0 tcontext=u:object_r:serial_device:s0 tclass=chr_file

Bug: 13506702
Change-Id: I3729537cabb0bf8e8b2905d3def43a293bb1081f