Add education tip to widget picker.

A tip is shown on the first widget/shortcut in the recommended table.
If there are no recommended widgets, a tip is shown on first widget
in an expanded header.

There is a delay of few milliseconds, to let the WidgetCells be
completely rendered on screen before getting their location.

Test: Manually tested
Bug: 184920163
Change-Id: I2637e84e7fc467b27888023434e3578a4b8ed4d6