Fix 6667238: allow market apps to support ACTION_ASSIST

This change allows market apps and 3rd parties to supply an activity
that responds to ACTION_ASSIST (e.g. market apps).

It also adds a test app to respond to the ASSIST intent and force
the intent disambiguation dialog to appear.

Change-Id: I5a78863c6a9546d18c66275187d178f6a1c9ee17