As Google's library for showing licences isn't open-source, this commit
reimplements its main Activity. This is in prevision to having an OSS
flavor of fenix.

We chose to not introduce dependencies to third-party libraries
such as AboutLibraries for now, and we'll stick to using Google's gradle
plugin for the dependencies extraction.

Fixes #7584

See also #162

Co-authored-by: Johan Lorenzo <jlorenzo@mozilla.com>

afaict, this is only used for BrowserPerformanceTestActivity, which is
being removed in this PR.

When we simplified our variants, our deep link scheme for nightly was
lost.

New API (installBuiltIn/ensureBuiltin) requires
- Gecko IDs and new permissions
- Extension will only be re-installed if it has a new version

This includes a gradle task to automatically generate a
new version in manifest.json for every build so we don't
forget to update the version and end up with changes that
are never applied.