PhoneWindowManager: add LineageButtons volumekey hook

We can't use a generic DeviceKeyHandler here, because we don't want to
override some of the AOSP logic (silencing incoming calls, screenshots)
and we need to know if the device is interactive or not (for screen-off
music controls)

Change-Id: I485c2f6006c5bbd358ba0cbd32917689826c5c6d