Don't live initialize i-ua

Live initialization in `_interaction_yes` modifier prevents some
other modifier (i.e. `platform`, `browser` etc) from being set during
initialization. Some CSS selectors match on them and if used along with
`i-ua_intearaction_yes` it causes page to appear differently before and
after click.
Allow to override `mix` mode of touch `b-page`

The way it was written it was impossible to override `b-page` mixes
on further levels.
Merge pull request #518 from bem/fix/no-live-ua

Don't live initialize i-ua