Prevent hole in ControlsActivity with panel

When the TaskView is created, until it displays the app content, it
shows a hole that shows the launcher. Instead, set its alpha to 0 and
only restore it to 1 when the task has finished creating.

Test: manual
Test: atest PanelTaskViewControllerTest
Fixes: 278923156
Change-Id: Ia3cd474e9888e57b258c985c520c3f08b09ee38d