Increase the maximum size for attachments.

It is often not possible to add more than two pictures, taken with a
current-gen camera with relatively high resolution, to an email. This is
because the maximum attachment size (for all attachments combined) defaults to
5MB when no user defined value is set. Since the option for setting the
attachment size is currently not implemented, let us at least default to
25MB (as in play store email apps for example).

See also:
https://code.google.com/p/android/issues/detail?id=65853

Change-Id: Ice1e79f7eb116c263178ffce891fba07fe051a76