Redesign for autocomplete result lists

[gnprice]

From @terpimost we have a new design for how the autocomplete result lists should be presented:

Details are in Figma:
https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=3954-13395&m=dev

The design within each item in the results is a separate related issue:

• Redesign for @-mention autocomplete items #913

This issue is about how the result list as a whole is presented:

• the fading effect at the top and/or bottom when scrolled
• the list's height, including not going off the top of the screen (I think we may already successfully avoid this)
• its rounded corners, border, and shadow
• its placement (dipping vertically a few pixels into the top of the text field, and aligning horizontally with the width of messages in the message list)
• any other details I'm missing right now

This should apply to autocomplete both for the content input, and for the topic input.