wm: Nullify hardkey function assignments if enabling the navbar

This caused erroneous (and sometimes duplicate) events being generated
due to the regular key function assignments. The navbar does its
own action management, so don't try to derive from the actions
usually present in hard keys.

Conflicts:
	policy/src/com/android/internal/policy/impl/PhoneWindowManager.java

Change-Id: I82866e24547f8145cac4f07820ae90aacce09281