Straighten out magic space vs real spaces behavior.

Picking a suggestion inserts a space after the word. This change makes
this space a magic space.

Incidentally, do some minor cleanup: add CODE_DASH, CODE_SINGLE_QUOTE
and CODE_DOUBLE_QUOTE to Keyboard and use them throughout the code,
and remove a useless import directive in some unrelated file.

Bug: 4319740

Change-Id: I245f396c34dd0af820bca91edc4ec363238a4ae4