Gallery2: if selected count is 0, leave selection mode.

If the selected count is 0, no need stay in selection
mode.

After we toggle a item, if selected count is 0，leave
the selection mode.

CRs-Fixed: 412492

Change-Id: Ia3ecd1c77f4074f21b1f906050b4a7bc2dc37c6e
Merge "Gallery2: if selected count is 0, leave selection mode." into du44