Add an ActivityManager callback for UID frozen state changes

Allow a client to ask for frozen state changes.  CachedAppOptimzer will
fire this callback when all processes in a Uid have become frozen and
also when a process in a fozen Uid becomes unfrozen.  Currently this is
intended to be used in ConnectivityService to kill sockets for frozen apps.

Freezing is done on a process basis however the callback operates on the
Uid level.  I added some methods and a boolean on UidRecord to query the
frozen state of it's proceses so that we only fire the callback when
appropriate.

This feature is off by default.

Test: Tested with a ConnectivityService patch to register for this callback
Test: and kill sockets on notification of a frozen Uid.
Bug: 253914114
Change-Id: I27ae40f345532836bdcb0e0dc3d7d7315c110d29