Fix headset play/pause/volume button functionality

The button jack is identified by the kernel as "Button Jack", so
Android looks for a "Button_Jack.kl" keylayout to handle the keycodes
from this device.

When it can't find it, it falls back to Generic.kl:

I/EventHub(  474): New device: id=7, fd=174, path='/dev/input/event8', name='Button Jack', classes=0x41, configuration='', keyLayout='/system/usr/keylayou

Rename msm8960-snd-card_Button_Jack.kl so Android can find it.

Change-Id: Ibf12ff9bd27a883da822fde3fc41c1d63200acdf