TaskOrganizer: Wait for apps to draw.

A Task may appear before the windows inside have finished drawing.
Rather than hand the burden to the TaskOrganizer of waiting for a
drawn signal, we simply defer emitting the leash until the draw
signal is received.

Bug: 152134500
Test: TaskOrganizerTests
Change-Id: I776b33fc637ac7f6d4ba2fe60b93cedf75ba19c0