Add Process.isProcessFrozen()

Add Process.isProcessFrozen(pid, uid).  The method returns true if
/sys/fs/cgroup/[apps,system]/uid_<uid>/pid_<pid>/cgroup.freeze is
readable and contains the character '1'.  Otherwise the method returns
false.  This means that the method returns false if the caller does
not have sufficient privileges.

The method queries the "system" path if the UID is less than 10000.
It queries the "apps" path if the UID >= 10000.

The command "isfrozen <PROCESS>" is now avaiable in the am shell
command.  It prints "true" or "false" depending on the return value
from the new method.

Flag: EXEMPT bugfix
Bug: 409803220
Test: manual test using 'am' commands to freeze, unfreeze, and query
processes.

Change-Id: I27d8a82716d593d4e714e375d79a400381765fa8