Handle user removed for PackageMonitorCallbackHelper

Currently, when a user is removed from the system, the caller doesn't
call unregister method. Even though the registerd clients are mostly
use UserHandle.ALL now but we still need to handle it correctly to
avoid leakage.

Bug: 29385425
Test: manual. Create a user and remove user to make sure the method
works as expected.

Change-Id: Ibcdcdf29d552b5b0ea6ac877ce3956a900362870