Use `environment.uniqueId` with a fallback.

Background

As discussed in https://gitlab.com/eyeo/adblockplus/adblockpluschrome/-/issues/193#note_356942530, and to solve this use case https://gitlab.com/eyeo/adblockplus/adblockpluschrome/-/issues/193#note_355311848, where two filters on the same domain might interfere with each other if not inlined/grouped as single one, with multiple snippets, we'd like to use a uniqueId whenever it'll be available through the environment object, so that nothing would be secured more than once, whenever snippets get injected on the user page.

What to change

Set the current uniqueIdentifier as environment.uniqueId, and fallback with a unique value.

Integration note

The extension should provide a random token on the evironment object that is initialized during extension startup and remains consistent while the extension is loaded per domain and browser session, or in any other way that would prevent malicious scripts from tracking, as discussed in https://gitlab.com/eyeo/adblockplus/adblockpluschrome/-/issues/193#note_358535427 /cc @snoack