Session Transfer Uri API Changes

Makes the following changes to the session transfer Uri API:
1. Changes the API from an extra to a setter/getter method in order to
   match AssistContent#setWebUri.
2. Renames the URI to AssistContent#setSessionTransferUri (removing the
   word web) in order to prevent developers from believing that this Uri
   will help with authentication.

Bug: 384877505
Flag:com.android.window.flags.enable_desktop_windowing_app_to_web_education
Test: atest DesktopModeWindowDecorationTests
API-Coverage-Bug: 388044324

Change-Id: Ia3eddca7bb61102b0e80c97667b7d2b8d0eeb4db