The user has to specify the language they're posting in, and sometimes
they might get it wrong (e.g., replying to a post that also had the
language set incorrectly, forgetfulness, etc).

This has accessiblity issues (only following statuses in a given
language fails, translation can fail, etc).

Prevent this by trying to detect the language the status is written in
when the user tries to post it. If the detected language and the set
language do not match, and the detection is 60+% confident, warn the
user the status language might be incorrect, and offer to correct it
before posting.

This is currently implemented using Google's ML Kit, and therefore only
available in the `google` store flavour.

To do this:

- Add `LanguageIdentifier`, with methods to do the identification,
  and `LanguageIdentifier.Factory` to create the identifiers.
- Inject the factory in `ComposeActivity`
- Detect the language when the user posts, showing a dialog if there's
  a sufficiently large discrepency.

The ML Kit dependencies (language models) will be installed by the Play
libraries, so there's some machinery to check that they're installed,
and kick off the installation if not. If they can't be installed then
the language check is bypassed.

Update the privacy policy, as the MLKit libraries may send some data
to Google.

- Move the language identifiers for API 29 and NOP in to the primary
  code, as they don't have dependency issues.

- Provide a default language detector factory for all devices.

- Return Result<> from language detection to cover possible exceptions.

- Pay attention to trying to use the language detector after close.