Update gBoard icon to Go/Search depending on suggestions.

If there is a highlight item, show Go (->) icon. Otherwise, show Search (magnifying glass) icon.

Implement maybeUpdateEnterKeyIcon() in NexusLauncher because it's gBoard specific.

Test: manual. Verify icon changes according to states.
Bug: 183467628
Bug: 210661150
Change-Id: I5dbcb54a37f9d37ae31bf3eaef241ea8a3ede510