Change UpdateRateControl() to return bool

Change UpdateRateControl() to return bool

Change the VP9RateControlRtcConfig constructor to initialize
ss_number_layers (to 1).

Change UpdateRateControl() to return bool so that it can report failure
(due to invalid configuration).

Also change InitRateControl() to return bool to propagate the return
value of UpdateRateControl().

Note: This is a port of the libaom CL
https://aomedia-review.googlesource.com/c/aom/+/172042.

Change-Id: I90b60353b5f15692dba5d89e7b1a9c81bb2fdd89