Search further than one task for fullscreen.

When a non-fullscreen task over home launches another non-fullscreen
task then the home task might not be displayed. Looking all the way
down the task stacks until reaching a visible, fullscreen activity or
home provides the right information.

Fixes bug 11273803.

Change-Id: I8dab0956c1cda06ddb7850ea3ffac7f6a223c6ad