vp9-rtc: Move noise estimation to after scene change detection

vp9-rtc: Move noise estimation to after scene change detection

This allows to use result from scene chage detection to exclude
the current frame from noise estimation analysis if the frame has
scene/ big content change (i.e., high_source_sad flag is set).

The behavior change for noise estimation may be small in practice,
since in the current code, a scene change would have blocks excluded
due to thresh_sum_diff, and the subsequent frames would also be mostly
excluded due to (past) non-zero motion vectors (until the
consec_zeromv > thresh_consec_zeromv is satisfied again).
But its better to completely exclude current frame if its a scene change.

Change-Id: Icd08bab7a8e1b994c7accced89697e0b2d7f50c5