autoadapt: improve algorithm parameters

- Reduce time during bitrate increase (to reach bandwidth limit faster)
- Increase the default bitrate value from 600 kbit/s to 800 kbit/s
- Increase maximum bitrate value from 3 Mbit/s to 6 Mbit/s (needed for high resolution, if we use a low resolution the CRF mode will lower the bitrate anyway)

Change-Id: I467649d27178e5f8577d313f3d12ce0ed6712005