Move long screenshot UI to an Activity

Separate UI code from image acquisition.
Using a static variable to get scroll capture connection from the
Activity, can do something more elegant later.
Doesn't do full activity restoration yet.

Bug: 179906912
Test: Running through long screenshot flow, turning screen on/off,
returning to task.

Change-Id: If5715c7008bbf3a1c8918a202a733fb3f8e8f876