Use layer dependent gfu_boost factor

Use layer dependent gfu_boost factor

When multi-layer ARF is enabled, use the corresponding gfu_boost
factor assigned to each ARF to compute the best_quality_index
adjustment. This on average improves the coding performance by
0.2% for lowres and hdres, 0.4% for ntflx2k. It seems this change
will only affect a small group of clips, e.g., pamphlet, bowing,
mobcal_720p, etc., which tend to gain 4-5%, whereas the rest
clips remain largely identical coding statistics.

Change-Id: Ie19636a6cf32214aefd73e21ead2aea647ddbca8