Clean up task affinity.

If activity is launched through external intent, back button should just
go back to previous screen.

Change-Id: I5b2e57d2751f5ed7c17140d3eb8d54736b78053c
Merged-In: I5b2e57d2751f5ed7c17140d3eb8d54736b78053c
Bug: 112254258
Fixes: 111864355
Test: manual