Ensure arraylist index is montonically decreasing

A previous fix for IndexOutOfBoundsException (ag/584621) left a
situation where the index would not decrement if the arraylist
size didn't change. The size doesn't change if the window being
removed is animating away. That caused window manager to remain
in an infinite loop within removeAllWindows.

This change ensures that the index diminishes each pass through
the loop and doesn't exceed the bounds of the arraylist.

Fixes bug 18362246.

Change-Id: Ibca70d95622f3b152ede14857f0e913099dc7b88
diff --git a/services/core/java/com/android/server/wm/AppWindowToken.java b/services/core/java/com/android/server/wm/AppWindowToken.java
index 1086eb2..f859fd2 100644
--- a/services/core/java/com/android/server/wm/AppWindowToken.java
+++ b/services/core/java/com/android/server/wm/AppWindowToken.java
@@ -254,15 +254,18 @@
 
     @Override
     void removeAllWindows() {
-        int winNdx;
-        while ((winNdx = allAppWindows.size()) > 0) {
-            WindowState win = allAppWindows.get(winNdx - 1);
+        for (int winNdx = allAppWindows.size() - 1; winNdx >= 0;
+                // removeWindowLocked at bottom of loop may remove multiple entries from
+                // allAppWindows if the window to be removed has child windows. It also may
+                // not remove any windows from allAppWindows at all if win is exiting and
+                // currently animating away. This ensures that winNdx is monotonically decreasing
+                // and never beyond allAppWindows bounds.
+                winNdx = Math.min(winNdx - 1, allAppWindows.size() - 1)) {
+            WindowState win = allAppWindows.get(winNdx);
             if (WindowManagerService.DEBUG_WINDOW_MOVEMENT) {
                 Slog.w(WindowManagerService.TAG, "removeAllWindows: removing win=" + win);
             }
 
-            // {@link WindowManagerService.removeWindowLocked} may remove multiple entries from
-            // {@link #allAppWindows} if the window to be removed has child windows.
             win.mService.removeWindowLocked(win.mSession, win);
         }
     }