Inprove database dump

- Use the best compression ratio.

- Report the correct file size via DumpFileProvider, so that Gmail
will be able to show an error if the file is too large. (gmail has ~25M limit.)

In case it's too large, we can ask the user to use Drive instead.

Change-Id: I625bf8913aa6fc2fce25130a219720b8d1a8896b
2 files changed