System bar now renders the back button for recents

We used to render the back button in Recents
itself, but that had an inconsistent visual
experience.

As a part of this change, add the early beginning
of support for private status bar APIs for dimming
navigation buttons.

Bug: 3448536
diff --git a/core/java/android/app/StatusBarManager.java b/core/java/android/app/StatusBarManager.java
index 5b8addf..dd9f337 100644
--- a/core/java/android/app/StatusBarManager.java
+++ b/core/java/android/app/StatusBarManager.java
@@ -56,6 +56,11 @@
             | DISABLE_NOTIFICATION_ALERTS | DISABLE_NOTIFICATION_TICKER
             | DISABLE_SYSTEM_INFO | DISABLE_RECENT | DISABLE_HOME | DISABLE_BACK | DISABLE_CLOCK;
 
+    public static final int NAVIGATION_HINT_BACK_NOP      = 1 << 0;
+    public static final int NAVIGATION_HINT_HOME_NOP      = 1 << 1;
+    public static final int NAVIGATION_HINT_RECENT_NOP    = 1 << 2;
+    public static final int NAVIGATION_HINT_BACK_ALT      = 1 << 3;
+
     private Context mContext;
     private IStatusBarService mService;
     private IBinder mToken = new Binder();