Makes gesture back event sends to the secondary container

In ActivityEmbedding, performs back gesture on the primary container
finishes the activity on the primary container and performs back
gesture on the secondary container finishes the activity on the
secondary container.

With this CL, it is always the activity on the secondary container
be finished, regardless the back event is done from which side the
swipe is done.

Bug: 240575809
Test: verified on Settings
Test: atest BackNavigationControllerTests
Change-Id: I6d5df6045424c4234e153012fa4cb84a8c990d39