Fix drag notification to split

On drag notification to split case, the pending intent calling Uid
should be app's Uid and lead to this start failed due to not system
and app didn't visible.
Put allow background activity flag on split side to ensure it could
start successful and active split screen.

Fix: 234406935
Test: manual
Test: pass existing tests
Change-Id: Ib52023f6d79d166b2b01948f2481350280a094f3