Adding internal method replacePreferredActivity.

This was required because we need a way to set the preferred activity for a
particular intent filter based on user selection (in our case the
ACTION_WEB_SEARCH intent filter for selecting the preferred search engine from
the list of available search engines providers). The current
addPreferredActivity call was not sufficient since it leaves the existing
preferred activities in the list and does not remove them, which this call
does.