SystemUI: Use correct back drawable for 2-button nav

<?xml version="1.0" encoding="utf-8"?>

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     Copyright (C) 2018 The Android Open Source Project

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<vector xmlns:android="http://schemas.android.com/apk/res/android"

    android:width="28dp"

    android:height="28dp"

    android:autoMirrored="true"

    android:viewportWidth="28"

    android:viewportHeight="28">

    <path

        android:pathData="M16.78,10.03l-3.97,3.97l3.97,3.97l-1.06,1.06l-5.03,-5.03l5.03,-5.03z"

        android:fillColor="?attr/singleToneColor" />

</vector>

    }

    public KeyButtonDrawable getBackDrawable() {

        KeyButtonDrawable drawable = getDrawable(R.drawable.ic_sysbar_back);

        KeyButtonDrawable drawable = mShowSwipeUpUi

                ? getDrawable(R.drawable.ic_sysbar_back_quick_step)

                : getDrawable(R.drawable.ic_sysbar_back);

        orientBackButton(drawable);

        return drawable;

    }