Make ChooserActivity intents doccentric and transitory

Activities launched from the chooser activity will now appear in
their own tasks which will be automatically removed from recents
when they are finished.

Also qualified application of new flags with null check and
Action check. Must be either ACTION_SEND or ACTION_SEND_MULTIPLE.

Fixes bug 14463859.
Fixes bug 14633773.

Change-Id: I8832462163958f6a43bc4c6a020f78948ce70ac3