Use --tune=[cdef-dist|daala-dist] to enable them.

Also, this commit set the use_activity_masking of PVQ as 0 by deafult,
which means that PVQ assumes daala-dist is not used by default.

Since we're currently not signaling which metric the encoder did use
in the bitstream, the compile flag AV1_PVQ_ENABLE_ACTIVITY_MASKING will tell PVQ
whether daala-dist is used or not.

This commit is the last part of prep-work to remove DIST_8X8, CDEF_DIST,
and DAALA_DIST experimental flags.

Change-Id: Ia465b4d6fe64aac7f04852c8f9f4bac3409d2435