libsnapshot: Fix premature truncation in CowWriter.

When opening in append mode, we ftruncate() the COW. This has three side
effects:
 (1) If the COW is never modified, or Finalized(), the state of the COW
     will have changed. Ideally it should only change on an explicit
     write operation.
 (2) Data after the current cluster will be accidentally thrown away.
 (3) The ending "cluster" op will be thrown away if the current cluster
     was incomplete, and thus the last valid label could be invalidated.

Bug: 183985866
Test: cow_api_test
Change-Id: I3c9a38553b7492a3d6e71d177d75ddb1b6490dfe