Add PendingIntent extra for WEB_SEARCH intents

This allos apps (e.g. browsers) that launch web searches
to control how the search results page is opened. For example,
the Android Browser will use this to make sure that searches
typed in the browser address bar open in the Andorid Browser,
and not somewhere else.

Bug: 4988580
Change-Id: I4665c9812ca8d704adbaea22a3812188bb48d1d2