Make default lag-in-frames=19
Make default lag-in-frames=19

The earlier default was 25, which was reduced to 17 in
https://aomedia-review.googlesource.com/c/aom/+/28181

However, as temporal filtering may use 3 frames on either side by
default, we should have default lag in frames as 16 + 3 = 19.

The compression performance is exactly neutral between lag-in-frames of
25 and 17.

Change-Id: I5603f899183a0363eb4c9160a6cacc4e68585725

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom