fix(adm): Filter keywords that do not start with the partial query

[ncloudioj]

This fixes #242.

Those suggestions, reported in #242, all have a wide range of targeting keywords, e.g

[
  "cars eb",
  "cars eba",
  "cars ebay"
  "eb",
  "eba",
  "ebay",
  ...
]

Hence, we need to filter out those keywords that don't share the same prefix with the search query (e.g. "eb") when trying to extract the full keyword.

[Dexterp37]

This looks good to me. Where is the same implementation that lives in Firefox?

[ncloudioj]

This looks good to me. Where is the same implementation that lives in Firefox?

Here it is: https://searchfox.org/mozilla-central/source/browser/components/urlbar/UrlbarQuickSuggest.jsm#160

@Dexterp37 Thanks for the review!