Fix NPE with inPurgeable Bitmaps in getAllocationByteCount

bug:11556330
Change-Id: I430136ea15baeb60adfb9bb2acdde9505ce707fe

graphics/java/android/graphics/Bitmap.java

@@ -1167,6 +1167,11 @@ public final class Bitmap implements Parcelable {
      * @see #reconfigure(int, int, Config)
      */
     public final int getAllocationByteCount() {
+        if (mBuffer == null) {
+            // native backed bitmaps don't support reconfiguration,
+            // so alloc size is always content size
+            return getByteCount();
+        }
         return mBuffer.length;
     }