Fix flaky RadioButton test testToggleViaEmulatedTap()

Fixes: 187796424

RadioButton clicks through touch input will post the onClick()
after the touch has happened. We must wait for the posted message
to be received before expecting any reaction.

Test: ran testToggleViaEmulatedTap
Change-Id: Ie9157f5ecd56a21eed3a1a6dd750e4ecbf6adbfa
1 file changed