Preserve tab position

To reproduce the issue:
1. Swipe to work tab
2. Tap Home
3. Swipe up to show all apps, you can see the personal tab is
   highlighted while the view pager is showing work apps.

We cleared the state whenever onLayout is called, we should
make use of mSelectedPosition.

Change-Id: I621b3c9ada0eb608280fe5e4ea8715f3a341820d

src/com/android/launcher3/views/SlidingTabStrip.java

@@ -34,7 +34,7 @@ public class SlidingTabStrip extends LinearLayout {
     private int mSelectedIndicatorHeight;
     private int mIndicatorLeft = -1;
     private int mIndicatorRight = -1;
-    private int mSelectedPosition = -1;
+    private int mSelectedPosition = 0;
     private float mSelectionOffset;
 
     public SlidingTabStrip(@NonNull Context context, @Nullable AttributeSet attrs) {
@@ -63,8 +63,8 @@ public class SlidingTabStrip extends LinearLayout {
     @Override
     protected void onLayout(boolean changed, int l, int t, int r, int b) {
         super.onLayout(changed, l, t, r, b);
-        updateTabTextColor(0);
-        updateIndicatorPosition(0, 0);
+        updateTabTextColor(mSelectedPosition);
+        updateIndicatorPosition(mSelectedPosition, 0);
     }
 
     private void updateIndicatorPosition() {