Gallery: fix timeline page display empty

If timeline page is navigated to again, base MediaSet may be retrieved
from cache and its version will not change, causing endless reload()
calls. Use isLoading() to determine the state.

The regression is caused by I1e05d31632694692025ac78a2d83200281f464ab

Change-Id: Icb7d623bc04353bfbca6577d2734d1ef9494d2bc
CRs-Fixed: 984190