Do not show Home behind full screen activity

When ensureActivitiesVisibleLocked goes through foreground activity
stack and reaches non-fullscreen activity, it sets showHomeBehindStack
variable to true.

If there is a fullscreen activity behind, showHomeBehindStack remains
unchanged, which causes Home application to be displayed anyway.
In this case user will see a fullscreen activity and Home activity
simultaneously.

To fix the issue we set showHomeBehindStack to false when we reach
fullscreen activity in the activity stack.

This was made visible by the following commit:
446ef1de

Change-Id: I535c1283a4e26f5cf606375b837d4b7195324af0