VP8: disallow thread count changes

Currently allocations are done at encoder creation time. Going from
threaded to non-threaded would cause a crash.

Bug: chromium:1486441
Change-Id: Ie301c2a70847dff2f0daae408fbef1e4d42e73d4
(cherry picked from commit 3fbd1dca6a4d2dad332a2110d646e4ffef36d590)