Fix toolbelt disappearance on '@' input or empty input in keyword mode

This CL fixes two scenarios where the toolbelt could disappear from the
match list:
- When entering only '@' as omnibox input, toolbelt was cut from list
  for special keyword mode refresh behavior. Restoring to list, it was
  out of order because this special case input had special sorting
  logic. Fix is flag guarded and simpler/more-general than previous
  reordering logic.
- When entering any keyword mode with no additional input text, the
  toolbelt was excluded because the provider was initiating actual
  async requests and later clearing matches. Fix was to restrict this
  behavior to only '@page' mode as originally designed, but '@page'
  mode is not officially supported so more work may be needed to
  get toolbelt and '@page' mode working well together.

Bug: 424504345
Change-Id: I12e4e4c2250fded7058993fc16405d07c2e69394
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6680172
Commit-Queue: Orin Jaworski <orinj@chromium.org>
Reviewed-by: Orin Jaworski <orinj@chromium.org>
Reviewed-by: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1480696}