Follow-on fixes for long screenshot activity

- Add takeScrollCaptureConnection() instead of accessing the static var
    directly.
- Re-enable action buttons in the activity after export completes (fixes
    the flow where you go to share and then back out).
- Fix share intent to properly used EXTRA_STREAM.

Bug: 180756221
Bug: 180755704

Test: Take long screenshot, share and return to long screenshot
  activity, verify buttons still work. Share output to printer target,
  verify that content arrives properly.
Change-Id: I1a0b20df50e7a7b6415d623920b84b07cddb0c80