emulator: add wifi permission to emulator image

Right now, emulator does not have wifi support yet
but the wifi service is needed for the whole systemui
to not crash on internal master.

We have plan to enable wifi on emulator, and before
that we still want a bootable emulator image for
testing purpose.

Change-Id: I5fe2613d3678aa898b5f8ddf7875ab85dbd43314