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/ChooseLockGeneric.java

@@ -821,6 +821,14 @@ public class ChooseLockGeneric extends SettingsActivity {
         @Override
         public void onDestroy() {
             super.onDestroy();
+            if (mUserPassword != null) {
+                mUserPassword.zeroize();
+            }
+            // Force a garbage collection immediately to remove remnant of user password shards
+            // from memory.
+            System.gc();
+            System.runFinalization();
+            System.gc();
         }
 
         @Override

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

@@ -530,6 +530,19 @@ public class ChooseLockPassword extends SettingsActivity {
             }
         }
 
+        @Override
+        public void onDestroy() {
+            super.onDestroy();
+            if (mCurrentCredential != null) {
+                mCurrentCredential.zeroize();
+            }
+            // Force a garbage collection immediately to remove remnant of user password shards
+            // from memory.
+            System.gc();
+            System.runFinalization();
+            System.gc();
+        }
+
         protected int getStageType() {
             return mForFingerprint ? Stage.TYPE_FINGERPRINT :
                     mForFace ? Stage.TYPE_FACE :

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

@@ -617,6 +617,19 @@ public class ChooseLockPattern extends SettingsActivity {
             }
         }
 
+        @Override
+        public void onDestroy() {
+            super.onDestroy();
+            if (mCurrentCredential != null) {
+                mCurrentCredential.zeroize();
+            }
+            // Force a garbage collection immediately to remove remnant of user password shards
+            // from memory.
+            System.gc();
+            System.runFinalization();
+            System.gc();
+        }
+
         protected Intent getRedactionInterstitialIntent(Context context) {
             return RedactionInterstitial.createStartIntent(context, mUserId);
         }

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();

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