Implement CandidateView using HorizontalScrollView

This change re-implements CandidateView using HorizontalScrollView and
customized Button, so that the candidate strip can be flingable.

This change also introduces the following features.

- Highlighting candidate strip and space bar is configurable by
  @bool/config_candidate_highlight_enabled.
- Candidate strip can be centered with left/right padding if
  @dimen/candidate_strip_padding size is greater than 0.
- Candidate text size is configurable by @dimen/candidate_text_size.

Bug: 1575885
Bug: 3230726
Change-Id: Iedf78ff31bc9f2f6291a8e8fb7faa0e6b961dd6d