Fix: Heap memory leak while running network back2back view test

Bitmap with 6MB was getting created for AnimScreen and never freed.
JIRA-Fixed: SWE-3115

Change-Id: I9443c3eb90465d9263c6483a7693b48f96466282
diff --git a/src/com/android/browser/PhoneUi.java b/src/com/android/browser/PhoneUi.java
index a58cdac..5a656a2 100644
--- a/src/com/android/browser/PhoneUi.java
+++ b/src/com/android/browser/PhoneUi.java
@@ -71,6 +71,8 @@
     @Override
     public void onDestroy() {
         hideTitleBar();
+        // Free the allocated memory for GC to clear it from the heap.
+        mAnimScreen = null;
     }
 
     @Override