COW file size: better explain the exception size

The exception size was hardcoded and its explanation hidden in one of
the comments.
Move it to a separate constant e better explain why that is 64 * 2 / 8.

Bug: 176972301
Test: m
Signed-off-by: Alessio Balsini <balsini@google.com>
Change-Id: Ifcb527540882222916ada07dacf3f76f87609539