Add "hashtag: " to search suggestion oracle

The hashtags feature depends on the note DB, so "hashtag: " is only
added as a search suggestion when the note DB is enabled.

Change-Id: Ie0a05dd50843184a559b50df38cc72ed42f77533
Merge "Add "hashtag: " to search suggestion oracle"
Convert AbstractQueryChangesTest to use Google Truth

Change-Id: I8f3682879671d8b05f491308b99d5f45452ab320