Fix issue #17082301: replacePreferredActivity is ignoring userId

It was being given the argument and just...  ignoring it.

But the bulk of this change is to make replacePreferredActivity
better about replacing -- it now detects if the request will not
make a change and, in that case, just do nothing.

The reason for this?

It turns out that each time you install an app, the telephony
system is calling this function over 20 times to set the default
SMS app.  This is almost always doing nothing, but before this
change it means we would re-write packages.xml over 20 times...!

There are definitely more improvements that can be made here (delaying
write of packages.xml to allow them to batch together, reducing
the amount of calls being made), but until then this is a big
improvement.

Change-Id: I02c4235b8ecd5c13ef53e65d13c7dc2223719cec