Allow users to import a custom wake word model

[celskeggs]

While I can recompile Dicio myself to replace the wake word... it would be really nice if users could use a custom wake word more easily. This PR adds the ability to import a .tflite file to override the default downloaded "Hey Dicio" wake word.

Generating a custom wake word is out of scope for the app, but users can download a model from the Home Assistant collection or use the OpenWakeWord scripts for training on Google Colab.

Fixes #289 #275 #238

[Stypox]

I pushed a few commits which:

• fix some synchronization issues in the OpenWakeWord model
• avoid having the view model access the specific wake word device instance (which may be recreated at any time)
• make the methods to handle the custom model file static, so they can be used independently of the current wake word device
• make it so that the text in the notification changes from Listening for the "Hey Dicio" wake word to Listening for custom wake word automatically
• improve the strings a bit and add some suggestions on where to download the models

Other than that, thank you for this PR! The general approach was good, but I had to make some of the above changes to make the code modules more well separated and generalized.

Debug APK: https://github.com/Stypox/testing-apks/releases/download/13/app-debug.apk (can be tested with models downloaded from https://github.com/dscripka/openWakeWord/releases/ )