Don't retry kill if cgroups isn't available

It makes no sense, because there are no cgroup procs file.

Bug: 257264124
Test: atest MicrodroidBenchmarkApp
Change-Id: I4e3a118d2237afc46aa8fbcbad055afb7d56f464
diff --git a/libprocessgroup/processgroup.cpp b/libprocessgroup/processgroup.cpp
index 45ac99c..1da69ba 100644
--- a/libprocessgroup/processgroup.cpp
+++ b/libprocessgroup/processgroup.cpp
@@ -471,6 +471,11 @@
             *max_processes = processes;
         }
         LOG(VERBOSE) << "Killed " << processes << " processes for processgroup " << initialPid;
+        if (!CgroupsAvailable()) {
+            // makes no sense to retry, because there are no cgroup_procs file
+            processes = 0;  // no remaining processes
+            break;
+        }
         if (retry > 0) {
             std::this_thread::sleep_for(5ms);
             --retry;