vp9: Modify logic for flat blocks in nonrd-pickmode.

vp9: Modify logic for flat blocks in nonrd-pickmode.

For real-time screen content mode: when slide change
is detected, for spatially flat blocks (source_variance = 0) on
the re-encoded frame, skip inter modes (so force intra) if
non-zero temporal variance is detected for the coding block.
Add flag to keep track of re-encoded frame at max Q.
Reduces artifacts on slide change.

Change-Id: I28151f412aba6ab8cb03f30087c7ce16d443654b