Make ActionProxyReceiver use PendingIntent.

PendingIntent lets SaveImageInBackgroundTask pass the permission
directly to the target. Flow is:
  SaveImageInBackgroundTask (current user) ->
  ActionProxyReceiver (system user) ->
  Share or edit target (current user)

The pending intent created in the first step allows read/write
permission to be passed to the final destination, as ActionProxyReceiver
itself does not have access to the screenshot.

Bug: 159455848
Test: Share and edit a screenshot with the following variations:
      - lockscreen on / off
      - primary user / secondary user
      Verify that full UI flow apperars, final destination can show the
      screenshot
Change-Id: Ie92212f2ffc3f8a11c3e5380908460f5f7fbb439