Change widget host view background when giving focus to child.

Before, the FocusIndicatorView stayed on the widget host unless the
widget had its own focus change listener, which caused a couple of
problems, namely  (1) it was hard to know where the focus was from
a user's standpoint, and (2) the focus indicator could get stuck on
the screen if you entered a widget and then long-pressed workspace.
Now, the focus indicator goes away and instead the widget is framed
with a white border to indicate that the focus is inside it. The
frame goes away when pressing ESC or when touching the screen.

Bug: 26744224

Change-Id: I14025576fd7a9f901b4d969b878af0bebad79a70