Implementation of Models Manager and other changes

[niedev]

This PR implements the following features for the 3.0 version of RTranslator:

• Integration and optimization of bergamot translator for the firefox models: The support for 48 languages has been added.
• GUI and logic adaptations to manage models choice and models download: Created the new Model Manager page in the settings, with also a preview of the RAM usage for the selected settings; modified the initial download and configuration pages; improved the logic for the translation model switch; implemented the option to use Mozilla models only in the WalkieTalkie and Conversation modes.
• Downloader rebuilding to improve its flexibility for the new download manager of models: the new downloader is based on PRDownloader.
• Implementation of an audio queue to deactivate mic only when the TTS speaks in WalkieTalkie mode.
• Adaptations to new permissions requirements for APIs 36: Adapted the Services to the new permissions requirements; improved the permission request logic; added the possibility to use Conversation mode without the mic permission; created dialogs before permission requests that explain the purpose of them;
• Substitution of MlKit with an open source language detector: implemented a combination of FastText, Lingua and the Translation Dictionaries to identify a language with the same (or better) accuracy than the old MlKit method for the WalkieTalkie mode.

Other features implemented:

• Improved speech detection for the automatic mic in WalkieTalkie and Conversation mode with a mix of volume level (the old way) and a small neural network that detects voice activity (Silero VAD via Android VAD).
• Improved TTS for WalkieTalkie and Conversation mode: now each message has a button to start and stop the TTS for that message. (Thanks to @NikolaCavendish, PR feat: add replay TTS audio button in WalkieTalkie and Conversation modes #183).

If you want to test this version of the app early, download the beta here.