Fix chips library bugs related to setting the threshold to 1.

- Dropdown not being dismissed after having at least one chip and typing a character and deleting it.
- Selecting a chip, deleting it by pressing backspace, and entering another character brings the deleted chip back.

TESTED=chips on bigtop app

Change-Id: I09233ffa7dfe1f64ae9efe4a392fa84b64b8b10a