Re-use last available bitrate as an initial bitrate on next call.

When closing the peer connection the last available send bitrate is stored in localStorage.
When a new call starts the stored value is used as start bitrate via the x-google-start-bitrate fmtp parameter.
The previous behaviour was a very temporary quirk that doesn't really does what is intended to do. IMO there is not much value on keeping it.

If you insist then, I must argue that the old behaviour is maintained via a flag to set min video bitrate and not under a fake "set initial send bitrate".

What do you think?