When the apps moved from/to a VPN, a proxy broadcast is needed to
inform the apps that the proxy might be changed since the default
network satisfied by the apps might also changed.
Since the framework does not track the defautlt network of every
apps, thus, this is done when:
1. VPN connects/disconnects.
2. List of uids that apply to the VPN has changed.
While 1 is already covered by the current design, the CL implements
2 in order to fulfill the case that different networks have
different proxies.
Bug: 178727215
Test: atest FrameworksNetTests
Change-Id: Ifa103dd66394026d752b407a1bee740c9fcdad2b
53e8a267ab