Show time-based TTS progress bar

[gyim]

The TTS screen in v2.0 shows a progress bar that is based on the number of characters read. Because of this, it is only shown in the app, but not on the lock screen: the API of the lock screen (MPRemoteCommandCenter) expects time-based progress reporting.

To fix this, we should do the following:

• Export TTS output into an audio buffer
• Report progress to the lock screen
• Support time-based seeking