Easy app-installing for Guest users

Provides a helper to keep track of which admin-installed apps are chosen
for copying to another user and for installing them on that other user.

Test: atest SettingsLibTests:com.android.settingslib.users.AppCopyingHelperTest
Bug: 193281439

Change-Id: I1b4a2f66fafe698cc15ededbf8a07417e80d46c6