NavRing: Update search assistant drawable

If the user installs a 3rd party search assistant and chooses that
as the default action for the search assistant target, the drawable
should be updated.  This patch tracks the current search assistant
component and when the SearchPanelView is shown, the drawable is
updated if the default search assistant component has changed

Change-Id: I7c1770b66a0265c61b433759915db3f3094bd41b
REF: CYNGNOS-804