Rename Flush to Finalize

As we change to a more resumable format, flush mostly writes the final
parts of the file that are needed, which would write extra data that is
not needed to continue writing, and would immediately be overwritten.

Additionally, in the next patch we will fsync the file after adding an
op, making the flush built in, and the Finalize name more appropriate.

Change-Id: I1db48315dec097b0620a0dfe7e7fd0098610d482
Bug: 168829493
Test: builds