Add IIntentSender to ActivityContainer.startActivity

PendingIntents and IntentSenders can now be launched. Still does not
work once the host activity has been paused and resumed.

Window manager TaskStacks now exist independently of Displays and app
windows persist after Displays are removed below them. Attaching the
stack to a new Display does not yet restore the windows to it.

Fixes bug 12747909.

Change-Id: I509007ee23fda400b353f483cf6ecce08177763b