Use separate inter predictors for enc/dec

The decoder will construct inter predictor using lazy border extension,
while the encoder, going with multiple runs of motion search in the rate-
distortion optimization loop for each block, does border extension at
frame level. This commit makes separate the inter predictors for encoder
and decoder, respectively.

Change-Id: Ieca2fecba3a7201a6d64ef9f219e5d91e50559c3