Initial setup for widgets in Search

LiveSearchManager creates AppWidgetHost when user starts a new search session and destroys it when user returns to home. In addition, it also manages the creation and caching of PlaceholderSearchWidget which can be used to create AppWidgetHostViews.
Bug: 168321831
Test: Manual

Change-Id: I06a893028e55aa6e0702a4f1cd7a2edbb1f61671