ActivityManager API to check availability of app freezer

A method to verify the availability of the app freezer is required for
configuration code running in places like the Settings app

Bug: 155465196
Change-Id: I5779d263536091689a099eec0815f207dfbbf6ad
Test: verified its workings through the developer options CL

core/java/android/app/IActivityManager.aidl

@@ -673,4 +673,9 @@ interface IActivityManager {
      * @param state The customized state data
      */
     void setProcessStateSummary(in byte[] state);
+
+    /**
+     * Return whether the app freezer is supported (true) or not (false) by this system.
+     */
+    boolean isAppFreezerSupported();
 }

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

@@ -20171,4 +20171,15 @@ public class ActivityManagerService extends IActivityManager.Stub
             mUsageStatsService.reportLocusUpdate(activity, userId, locusId, appToken);
         }
     }
+
+    @Override
+    public boolean isAppFreezerSupported() {
+        final long token = Binder.clearCallingIdentity();
+
+        try {
+            return mOomAdjuster.mCachedAppOptimizer.isFreezerSupported();
+        } finally {
+            Binder.restoreCallingIdentity(token);
+        }
+    }
 }

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

@@ -407,7 +407,7 @@ public final class CachedAppOptimizer {
     /**
      * Determines whether the freezer is correctly supported by this system
      */
-    public boolean isFreezerSupported() {
+    public static boolean isFreezerSupported() {
         boolean supported = false;
         FileReader fr = null;