PhoneWindowManager: Conditionally register hardware key observers

This will only register the observers if mDeviceHardwareKeys != 0

next: poke a constant from here that will be read by settings to
remove the hardware key header from vanir settings xml if this
feature is unsupported. This will eliminate the need to add a config
entry in settings and use additional overlays since we already have
one in the frameworks

Change-Id: I8dfc4bfdbd4d03843018e3da8db621370c069a4f