Gallery2: Notify dirty after update content

The loading process end when the variable of 'mDirty' is false, and
this value will set false default and set true when NotifyDirty. As a
result, if the NotifyDirty called before set default value, the loading
process will end before update content.

Make sure there is a UpdateContent between two times of NotifyDirty.

Change-Id: Ie3ed56eba6a0e354f06436fabce4fca2810e28bd
CRs-Fixed: 789079