Gallery2: Add leave selection mode judgement

The method of isCancelled in thread pool is a nonsynchronous method,
as a result, there is a possibility that the job is not cancelled
after leave selection mode, and the getSelectedMediaObjects is still
running.

Finish getSelectedMediaObjects when selection mode is quit.

CRs-Fixed: 769028

Change-Id: Icb7c3f9d1f3e14536f32a6401337e844481ad646