Trigger error only when a template is instantiated.

Clang compiler will check defintions and trigger undefined template
error of 'android::CompileTimeAssert<false>' even when MixMul is not
instantiated. Avoid such parse time error but trigger link error when the
generic MixMul template is used.

Change-Id: I84ef60a69fbfbeefc93b5654e4bdd54f9d530867