Cleanup FeatureFlag of 2nd layer Activity navigate up button

After this change, navigate up button of 2nd layer Activity
will show/hide by SplitController#isActivityEmbedded.

Bug: 263198464
Test: build
      manual visual
Change-Id: I23b3b22693d1cd3b78a4c18f94f942285b9e5d07
2 files changed