Delay gargabe collection during onDestroy()

Allow the activity object to become unreachable before
iniating a gargabe collection to sanitize memory.

Bug: 189315376
Test: add device lock, confirm device lock in Settings,
      then a memory dump and search for password shards.
Change-Id: I807d74628e1355814807855ad309d86ca1fb38a1

src/com/android/settings/password/ConfirmDeviceCredentialBaseActivity.java

@@ -18,6 +18,8 @@ package com.android.settings.password;
 
 import android.app.KeyguardManager;
 import android.os.Bundle;
+import android.os.Handler;
+import android.os.Looper;
 import android.os.UserManager;
 import android.util.Log;
 import android.view.MenuItem;
@@ -163,11 +165,14 @@ public abstract class ConfirmDeviceCredentialBaseActivity extends SettingsActivi
     @Override
     public void onDestroy() {
         super.onDestroy();
-        // Force a garbage collection immediately to remove remnant of user password shards
+        // Force a garbage collection to remove remnant of user password shards from memory.
-        // from memory.
+        // Execute this with a slight delay to allow the activity lifecycle to complete and
+        // the instance to become gc-able.
+        new Handler(Looper.myLooper()).postDelayed(() -> {
             System.gc();
             System.runFinalization();
             System.gc();
+        }, 5000);
     }
 
     @Override

src/com/android/settings/password/ConfirmLockPassword.java

@@ -25,6 +25,8 @@ import android.graphics.Typeface;
 import android.os.AsyncTask;
 import android.os.Bundle;
 import android.os.CountDownTimer;
+import android.os.Handler;
+import android.os.Looper;
 import android.os.SystemClock;
 import android.os.UserManager;
 import android.os.storage.StorageManager;
@@ -216,11 +218,14 @@ public class ConfirmLockPassword extends ConfirmDeviceCredentialBaseActivity {
         public void onDestroy() {
             super.onDestroy();
             mPasswordEntry.setText(null);
-            // Force a garbage collection immediately to remove remnant of user password shards
+            // Force a garbage collection to remove remnant of user password shards from memory.
-            // from memory.
+            // Execute this with a slight delay to allow the activity lifecycle to complete and
+            // the instance to become gc-able.
+            new Handler(Looper.myLooper()).postDelayed(() -> {
                 System.gc();
                 System.runFinalization();
                 System.gc();
+            }, 5000);
         }
 
         private int getDefaultHeader() {