Allow WindowManager.TYPE_SCREENSHOT to be focusable

Pre-R, the screenshot window was used just for screenshot animation. In
R, the window is also hosting tappable screenshot actions, which require
focus.

This change modifies TYPE_SCREENSHOT to no longer force it to be
unfocusable.

Test: Verify that screenshot window UI elements can work with talkback
    and accessibility scanner.
Bug: 153517161
Bug: 152583784
Change-Id: If81d9f94dff801c3483a2d834e692b4c77d80d7b

core/java/android/view/WindowManager.java

@@ -1162,7 +1162,7 @@ public interface WindowManager extends ViewManager {
 
         /**
          * Window type: shows directly above the keyguard. The layer is
-         * reserved for screenshot region selection. These windows must not take input focus.
+         * reserved for screenshot animation, region selection and UI.
          * In multiuser systems shows only on the owning user's window.
          * @hide
          */

services/core/java/com/android/server/wm/DisplayPolicy.java

@@ -874,10 +874,6 @@ public class DisplayPolicy {
                 }
                 break;
 
-            case TYPE_SCREENSHOT:
-                attrs.flags |= WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE;
-                break;
-
             case TYPE_TOAST:
                 // While apps should use the dedicated toast APIs to add such windows
                 // it possible legacy apps to add the window directly. Therefore, we