Send APPWIDGET_UPDATE when font scale setting changes

Updates AppWidgetServiceImpl to update all of the widgets when the font
scale setting changes. Widgets often use the font scale to measure text
size when creating a layout, or they may draw text into a canvas to
create a custom bitmap. Since Launcher does not update the app widget
options when the font scale changes (since display size is the same),
we must update widgets manually so that they can relayout their text.

Bug: 434874426
Test: AppWidgetTest#testFontScaleChange
Flag: EXEMPT bugfix
Change-Id: I0e5da50ee1d03fd652a1a468d79246f325c3c36b