Add a timeout for removing starting windows.

If two activities are started at the same time the first activity can
add a starting window but never start. In that case there is no event
that will clear the starting window. This change adds a 10 second
timeout for the starting window to be cleared after which it will
clear the starting window automatically.

Fixes bug 10797865.

Change-Id: I1d59c3058c63367ff688d426474e8a6f006b2e0d