Gallery2: fix display abnormal when openning photo in MMS attachment.

Open photo in MMS attachment, then click "Share" button,
SinglePhotoDataAdapter.pause() is called, but mTask is not reset to null.

Reset mTask to null and
ensure to create new mTask instance in SinglePhotoDataAdapter.resume().

Change-Id: I278d22f04e22b8701e9f57da9bd5df8e485a19ad
CRs-Fixed: 963887