Fix leak of WeakReferences on mThemeRefs list.

Periodically remove references from the list whose referents have been
garbage collected.

Bug: 73961798
Test: Device boots.
Test: Take a heap dump of systemui and manually check that the state of
      ThemeRefs looks reasonable.

Change-Id: I691027feb5dd217bcb60406b28897b9614e2a845

core/java/android/content/res/Resources.java

@@ -127,6 +127,14 @@ public class Resources {
      */
     private final ArrayList<WeakReference<Theme>> mThemeRefs = new ArrayList<>();
 
+    /**
+     * To avoid leaking WeakReferences to garbage collected Themes on the
+     * mThemeRefs list, we flush the list of stale references any time the
+     * mThemeRefNextFlushSize is reached.
+     */
+    private static final int MIN_THEME_REFS_FLUSH_SIZE = 32;
+    private int mThemeRefsNextFlushSize = MIN_THEME_REFS_FLUSH_SIZE;
+
     /**
      * Returns the most appropriate default theme for the specified target SDK version.
      * <ul>
@@ -1770,6 +1778,13 @@ public class Resources {
         theme.setImpl(mResourcesImpl.newThemeImpl());
         synchronized (mThemeRefs) {
             mThemeRefs.add(new WeakReference<>(theme));
+
+            // Clean up references to garbage collected themes
+            if (mThemeRefs.size() > mThemeRefsNextFlushSize) {
+                mThemeRefs.removeIf(ref -> ref.get() == null);
+                mThemeRefsNextFlushSize = Math.max(MIN_THEME_REFS_FLUSH_SIZE,
+                        2 * mThemeRefs.size());
+            }
         }
         return theme;
     }