Wait for window focus in TimePickerTest

The TimePickerTest sends key events in order to select some things
inside the widget.
To make sure keys are received by the window of interest, wait for the
window to receive focus.

Bug: 151115878
Test: atest TimePickerTest
Change-Id: Ie23631d01c4aa95cf9cb59e5a854dc86437ea16a
3 files changed