Change widget host view background when giving focus to child.

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