This change also cleanups preference key string constants

Change-Id: I58510f220a90d00d71a935731abeab87384675bb

This change introduces new package com.android.inputmethod.keyboard
which hosts keyboard related classes.

Also adds missing @Override and @SuppressWarning("unused") annotation

Change-Id: I0317f5cfa317e13f3c77815f48ffc69b5d1ade05

Bug:3033472

Change-Id: I311c66ac3c612a30d7da3be8ee35a352f49016ac