LiveSession: raise upwards adaptation constraint

Adjust bandwidth more conservatively when considering an upwards
adaptation. Also fixed an issue with kWhatCheckBandwidth messages
being accumulated across switch generations; this causes
onCheckBandwidth to be fired at a high frequency and LiveSession to
be too sensitive to network glitches.

Bug: 13743153
Change-Id: I1dec99cb5d123c6675abe0847fd12aab5178eefd