CachedAppOptimizer: adapt freezer to uid/pid hiearchy.

The uid/pid hierarchy freezer is supported if the cgroup.freeze file is
present under any uid cgroup. Check on uid_0 which is granted to exist.

Also handle binder when enabling/disabling freezer. Freeze the binder interfaces
for all froze processes after enabling the app freezer. Similarly, unfreeze the
interfaces of frozen processes when the app freezer is enabled.

Bug: 168907513
Bug: 173173405
Test: verified proper functioning of the freezer after boot on devices
supporting the uid/pid hierarchy.
Test: ran "adb shell am dumpsys meminfo -a", verified that no processes
were killed by ActivityManager and that the command did not hang.

Change-Id: Ia7fa25cb6c59da73d34460ead4833cec69f7f6bc