Fix back button behavior for activities started from a notification

This change brings TextSecure closer to the behavior described in [1]. However,
the usage of RoutingActivity makes a slightly different approach necessary.
With this change RoutingActivity builds a new activity stack once the master secret
was retrieved and it's ready to launch ConversationListActivity or
ConversationActivity. This way pressing the back button when the password
prompt is displayed won't return to another instance of RoutingActivity that would
display another password prompt.

Fixes issue #480

[1]: http://developer.android.com/training/notify-user/navigation.html