libsnapshot: Remove Initialize(fd, APPEND)

All cases where we want to append are done via label, so we can remove
the far more complicated open mode for append. Since this only leaves
open, we can remove the mode entirely.

Bug: 172026020
Test: cow_api_test
Change-Id: Ibe305e56b434da25952a129764398f606ca58aff
5 files changed