Easy app-installing for Guest users

Allows an admin user to selectively install packages into a Guest user
by displaying a list of available packages that are installed in the
admin, but not in the Guest, and letting the admin choose to copy those
apps to the Guest.

Test: atest SettingsLibTests:com.android.settingslib.users.AppCopyingHelperTest

Test: Manual: install some apps in user 0, create a guest, uninstall
some system apps from it. Now, open this panel. Request some of those
apps be installed in the Guest and verify it worked.

Bug: 193281439

Change-Id: I4e6874a4ee93cd7bba96e1f6c8d04ed95873c1a2