Move BLOCKED_REASON_* constants from NPMS into ConnectivityManager.

These constants will now be including all the reasons for why an
uid's network access can be blocked, instead of only the
restrictions that could be imposed by NPMS.

Bug: 183473548
Test: atest ./tests/cts/hostside/src/com/android/cts/net/HostsideRestrictBackgroundNetworkTests.java
Merged-In: I4c544415e12adf442fd2415c371b1b70a39c3aa4
Change-Id: I6dcea43fbefa9eac8b5a971b822a5be5422a54b4