libprocessgroup: move freezer to uid/pid hierarchy

Migrate the freezer controller as an attribute for the uid/pid hierarchy

Bug: 168907513
Test: manually verified that the freezer behavior was as expected after
applying the enablers in ActivityManager

Signed-off-by: Marco Ballesio <balejs@google.com>
Change-Id: Ica2c4bfd20190fb2f2dc0a530eb47e1a8427c522