Collect final transcripts correctly in Android

[JamesBrill]

On Android, there are two strange behaviours as documented here:

• Interim transcripts are marked as "final" with confidence 0
• Final transcripts are often emitted twice

This PR handles this by doing the following:

• Treating "final" transcripts with confidence 0 as interim transcripts on Android
• Debouncing final transcripts on Android, waiting 250ms between each one