Add log to explain why freezer is not supported

The old log is too vague to identify what's wrong with the cgroup
freezer configuration. That makes debugging unnecessarily time
consuming. Improve the log with more details.

Bug: 269312428
Test: check logcat during Android boot
Change-Id: I7c2518aef003cca990ed321e22890bddde2c5d13

services/core/java/com/android/server/am/CachedAppOptimizer.java

@@ -941,11 +941,14 @@ public final class CachedAppOptimizer {
         FileReader fr = null;
 
         try {
-            fr = new FileReader(getFreezerCheckPath());
+            String path = getFreezerCheckPath();
+            Slog.d(TAG_AM, "Checking cgroup freezer: " + path);
+            fr = new FileReader(path);
             char state = (char) fr.read();
 
             if (state == '1' || state == '0') {
                 // Also check freezer binder ioctl
+                Slog.d(TAG_AM, "Checking binder freezer ioctl");
                 getBinderFreezeInfo(Process.myPid());
                 supported = true;
             } else {
@@ -967,6 +970,7 @@ public final class CachedAppOptimizer {
             }
         }
 
+        Slog.d(TAG_AM, "Freezer supported: " + supported);
         return supported;
     }