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/ConfirmLockPassword.java

@@ -217,6 +217,17 @@ public class ConfirmLockPassword extends ConfirmDeviceCredentialBaseActivity {
             }
         }
 
+        @Override
+        public void onDestroy() {
+            super.onDestroy();
+            mPasswordEntry.setText(null);
+            // Force a garbage collection immediately to remove remnant of user password shards
+            // from memory.
+            System.gc();
+            System.runFinalization();
+            System.gc();
+        }
+
         private int getDefaultHeader() {
             if (mFrp) {
                 return mIsAlpha ? R.string.lockpassword_confirm_your_password_header_frp