Change inspector auto complete so it allows quick navigation through search results

The need to revert https://github.com/flutter/devtools/pull/6677 highlighted that there is autocomplete behaviour that already uses the enter key in the search field.

To priotitize ease of use in the search field I propose that we:
- change index 0 of the selected indices to mean no selection
- allow enter, and shift+enter to navigate to next/previous while index 0 is set.
- In order to use "enter" to select an autocomplete entry, the user would have to use the down arrow once to enter the auto completions. This would mean that the autocompletions are essentially 1-indexed instead of 0-indexed.