libsnapshot: Add an append mode to CowWriter.

When in append mode, CowWriter will re-open the existing COW and resume
writing at the end of the old data position. All existing operations
will be reimported and buffered in memory.

The size calculation has been simplified to make this work. We now
advance ops_offset and no longer track the number of bytes written.

Additionally, a "header_size" field has been added to the header. This
was missing from the original format and is useful for introducing
forward compatibility later.

Finally, Finalize has been renamed to Flush. It's still mandatory, but
it can be called multiple times to continue appending data without
reopening.

Bug: 168554689
Test: cow_api_test gtest
Change-Id: I637e99ae08a4db5b273c06318e6db523ea8ec7c5