v2.0.7 bugfix on zero-input-pinyin

sometimes after GetCandidate dbus call, zero-input-fetch-size and
zero-input-candidates is not updated. This was because the async
function callback didn't update the metadata in some places.

- update initial fetch size and fetch size to be 1+ candidates needed for
  target page.
- removed zero-input-build-candidates-complete function,
  each async call should have its own callback function, don't reuse this one.
  in callback function, post GetCandidate metadata should be updated.