If you want only your main activity to be in visible your "recent apps", then in all your non-main activities, add this flag in your manifest:
android:noHistory
This will cause your main activity to remain on the activity stack, but remove all your activities from it.
http://developer.android.com/guide/topics/manifest/activity-element.html#nohist
Whether or not the activity should be removed from the activity stack
and finished (its finish() method called) when the user navigates away
from it and it's no longer visible on screen — "true" if it should be
finished, and "false" if not. The default value is "false". A value of
"true" means that the activity will not leave a historical trace. It
will not remain in the activity stack for the task, so the user will
not be able to return to it. This attribute was introduced in API
Level 3.