Enabled adaptive UV intra coding for inter frames

Enabled adaptive UV intra coding for inter frames

Previously, Y-adaptive UV intra coding only enabled on key frames in
UVINTRA experiment. This commit enabled the same coding for inter
frames, so the encoding of UV intra modes are consistent cross all
frame types. Tests on derf set showed a very small overall gain around
.04%:

http://www.corp.google.com/~yaowu/no_crawl/interUVintra.html

The gain looks to be reasonable given inta coded MBs is only a
small portion of MBs in inter frames.

Change-Id: Ic6fc261923f2c253f4a0c9f8bccf4797557b9e16