httplive: Defer switch down if a switch is in progress

Bandwidth switch down is triggered if the buffered duration in
any of the current packet sources is below a threshold. When a
switch is in progress, all the packet sources are drained until
they are empty or until stop time is dequeued. Hence buffered
duration keeps going down during switch. Defering check switch
down will avoid unnecessary switches.

Do not switch down if estimated bandwidth index is more than
the current one.

Bug: 18821145
Change-Id: I655a308462503cf9df10672ecd904a51b2cba691