Stabilize the encoder buffer from going too negative.

For screen-content mode, with frame dropper off, put a limit
on how low encoder buffer can go.

Under hard slide changes, the buffer level can go too low and then
take long time to come back up (in particular when frame-dropping
is not used), which will affect the active_worst and target frame size.

Change-Id: Ie9fca097e05cd71141f978ec687f852daf9de332