run full explicit GC in SystemUI and system_server after locking

ART has special support for System.gc() and Android already uses this to
purge memory after unlocking. It makes sense to purge as much as we can
when locking too.

packages/SystemUI/src/com/android/systemui/keyguard/KeyguardViewMediator.java

@@ -2769,6 +2769,9 @@ private void handleShow(Bundle options) {
             userActivity();
             mUpdateMonitor.setKeyguardGoingAway(false);
             mKeyguardViewControllerLazy.get().setKeyguardGoingAwayState(false);
+            System.gc();
+            System.runFinalization();
+            System.gc();
             mShowKeyguardWakeLock.release();
         }
         mKeyguardDisplayManager.show();

services/core/java/com/android/server/policy/keyguard/KeyguardStateMonitor.java

@@ -97,6 +97,12 @@ public void onShowingStateChanged(boolean showing, int userId) {
                     ExtSettings.DENY_NEW_USB_TRANSIENT_ENABLE :
                     ExtSettings.DENY_NEW_USB_TRANSIENT_DISABLE);
         }
+
+        if (showing) {
+            System.gc();
+            System.runFinalization();
+            System.gc();
+        }
     }
 
     @Override // Binder interface