Android: Voice typing: Improve silence detection

[personalizedrefrigerator]

Summary

This pull request increases the tolerance for silence detection both when splitting the recorded audio buffer and when determining whether the full audio buffer is silent.

This partially resolves an issue raised while increasing the default voice typing model size.

Testing plan

Manual testing on Android 13:

1. Start voice typing and read the first paragraph from https://en.wikipedia.org/wiki/Four_color_theorem.
2. Wait 1-2 seconds.
3. Click "Done".
4. Verify that the first paragraph (and no additional unrelated lines) are transcribed.