Clean up apks installed for a removed user

When a user is removed, enumerate through all installed packages
to see if any of them are not installed for any user. Delete the
package if no user has it "installed".

Added a pm option to install an apk for a specific user.

Fixed a crash in UserManagerService when executing the above
cleanup - dying users generate a null UserInfo.

Bug: 15426024
Change-Id: I571decde1ae1c257d0da6db153b896aad6d6bcb4