Using actual users in more places.

Fixing an issue when installed apps in Guest user were treated incorrectly.
This happened because we used APIs using the systemui current user (0),
not the app's user.

Also, switching AppInfo from user's serial numbers to UserHandle. Earlier,
I declared that user ids (not serials) is the "lingua franca" between different classes,
so, this move is along these lines. In addition, this is simply more convenient.

Bug: 20024603
Change-Id: I749d900aa17083a41e4215c6587ae5ccaa31f5eb