Attempt to clear memory after keyguard PIN entry

Force object finalization and garbage collection to remove shards
of user PIN from memory.

Bug: 189315376
Test: Unlock keyguard; take ramdump; search for PIN in the dump
Change-Id: I6d951babc225998d959669117c26f823f28fa706

packages/SystemUI/src/com/android/keyguard/KeyguardSecurityContainerController.java

@@ -146,7 +146,9 @@ public class KeyguardSecurityContainerController extends ViewController<Keyguard
                     try {
                         Thread.sleep(5000);
                     } catch (InterruptedException ignored) { }
-                    Runtime.getRuntime().gc();
+                    System.gc();
+                    System.runFinalization();
+                    System.gc();
                 });
             } else {
                 SysUiStatsLog.write(SysUiStatsLog.KEYGUARD_BOUNCER_PASSWORD_ENTERED,