Add IN_BACK_MIC to input devices

This allows things like echo cancellation and noise filtering to actually work by enabling the secondary microphone.

When I added an Otterbox Defender case to my Note 2, my wife noticed that she heard an echo of everything she said to me during a call. I did some looking, and it turns out there were several complaints of this, and mention that a software update from AT&T fixed it. So, I had a hunch and took a look at the audio_policy.conf file in AT&T's newest stock ROM.  Sure enough, it has the back mic enabled, and ours doesn't.

I made this change to my phone, and rebooted. My wife then reported that the nasty echo was gone.

Qualifier:  I have only tested this on one call, and while the results look promising, I can't actually guarantee that they work across the board. I can, however, say that at the very least, this change does no harm.