Gallery2: enhance performance of update supported operation.

getSelectedMediaObjects() in updateSupportedOperation() is low performance
if there are lots of photos, so menu options will not in correct state
immediately, then crash occurred if click menu option.

Use mSelectionManager.getSelectedCount() before getSelectedMediaObjects()
to disable menu options and share items if need.

Change-Id: I937a219ed294151c464f1d7b6ba2e526eef00c82
CRs-Fixed: 985315