Clear sender images when dimensions change.

When switching to two-pane layout, the dimensions of sender images change. This
caused a bug where we cleared the sender image bitmap, but failed to update the
state to reflect that. What resulted was an empty bitmap being drawn to the
screen.

Bug: 9885718
Change-Id: I6e421fe47c52ee940b994369e92aa0bc3d057327
diff --git a/src/com/android/mail/ui/DividedImageCanvas.java b/src/com/android/mail/ui/DividedImageCanvas.java
index b45e88b..274327e 100644
--- a/src/com/android/mail/ui/DividedImageCanvas.java
+++ b/src/com/android/mail/ui/DividedImageCanvas.java
@@ -418,10 +418,13 @@
         mWidth = width;
         mHeight = height;
 
-        // todo:ath this bitmap is creating a GC which is killing CIV.loadAttachmentPreviews()
         mDividedBitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
         mCanvas = new Canvas(mDividedBitmap);
-        mBitmapValid = true;
+
+        for (int i = 0; i < getDivisionCount(); i++) {
+            mDivisionImages.set(i, null);
+        }
+        mBitmapValid = false;
         Utils.traceEndSection();
     }