Fix 6592932: add means to replace assist icon from given package

This provides the means to replace the assist icon shown in keyguard and the
navigation gesture for assist.  It's done by adding metadata called
"com.android.systemui.action_assist_icon" to the activity that handles
android.intent.action.ASSIST.  It should point to a StateListDrawable
in that package with the required states.  For example:

<meta-data android:name="com.android.systemui.action_assist_icon"
    android:resource="@drawable/ic_android_systemui_action_assist" />

And then something like this in drawable/ic_android_systemui_action_assist.xml :

<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_enabled="true"
        android:state_active="false"
        android:state_focused="false"
        android:drawable="@drawable/ic_action_assist_normal" />
    <item android:state_enabled="true"
        android:state_active="true"
        android:state_focused="false"
        android:drawable="@drawable/ic_action_assist_activated" />
    <item android:state_enabled="true"
        android:state_active="false"
        android:state_focused="true"
        android:drawable="@drawable/ic_action_assist_focused" />
</selector>

Change-Id: Ibc29360e179fed68253ff06a07b1bb2b982d0dab