DownloadProvider: fix the crash when rotate screen.

TrampolineActivity will be recreated when rotate screen,
then the fragment is currently associated with any Activity.
So getActivity() will return a null and caused NullPointerException.

Add the null check before call finish().

CRs-Fixed: 640914

Change-Id: I76f368a597552e824d533b6a66ea23fad4c0cb41