Fixed issue with prematurely removing display for child activities.

When relaunching a parent activity we finish all its children and set
a 2sec timeout to clean up their tasks/stacks/displays if we don't
hear anything back. This time might not be sufficient for the client
activity to respond back in time depending on what else is happening
on the system and we might end-up prematurely removing its display
which will cause it to crash. In this specfic case, the client
activity which wasn't the focus activity was been relaunched with a
bunch of other activities due to a configuration change.

Instead of having a timeout for the clean-up we now let the activity
go through the normal clean-up process that occurs as it changes
states which will eventually clean-up the right states in a max.
time of 10secs (on destroy timeout) or sooner. This is in line with
activity cycle expections.

Bug: 17702043
Change-Id: I484124e07ad32b9056f75ec41af1dd7718488335