UI adjustment to RTT screen.

1. Don't close overflow menu after menu item is clicked.
2. Add correct shadow and animation to overflow menu.
3. Only scroll when remote message is updated if input box is empty.
4. Only hide keyboard if scrolling up is triggered by user.
5. Add paddingTop to recyclerView so that first message won't be covered by toolbar.

Bug: 70177426
Test: manual
PiperOrigin-RevId: 191002774
Change-Id: Ic69f80323e3d31d7853d4e8d0c6e4c100fe4a776