Reland "Add an API to listen for changes in network blocked status of an uid."

This reverts commit 8623f2b3.

Reason for revert: The issue causing the build breakage has been fixed

Change-Id: I03fa406551b51aaa4d4d9255cf1a53f9b4bcc1bd
Merged-In: Ib9949b8619c6b148f73630b314c4113d76c31ec1