Remove password shards from memory

Force a garbage collection and zeroize some fields after Activity finishes

Test: Goes through password change flow, then grab a heap dump via
      adb shell 'am dumpheap $(pidof com.android.settings)
      /data/local/tmp/settings.hprof'
      And grep for password in the dump
Bug: 144537463
Change-Id: Idd0a04ada98900aeb2a6d20bb1270a4a4aec2cfd

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

@@ -157,6 +157,16 @@ public abstract class ConfirmDeviceCredentialBaseActivity extends SettingsActivi
         }
     }
 
+    @Override
+    public void onDestroy() {
+        super.onDestroy();
+        // Force a garbage collection immediately to remove remnant of user password shards
+        // from memory.
+        System.gc();
+        System.runFinalization();
+        System.gc();
+    }
+
     @Override
     public void finish() {
         super.finish();