Make InputBindResult#accessibilitySessions final

This field was somehow added as a non-final field [1], but nothing
prevents this from being a final field.  Let's make this final to be
consistent with other fields in InputBindResult.

This is a safe refactoring.  There should be no observable behavior
change.

 [1]: I8efdc9886ce33185a2195b741668c12e319ea660
      107b9413

Bug: 234882948
Test: presubmit
Change-Id: I5aef2e6a744f435fd192590673e91cb26cc3b02c