Don't throw away wallpaper bitmap just because we are hidden.

TRIM_MEMORY_UI_HIDDEN > TRIM_MEMORY_RUNNING constants, so we only
need to throw away the wallpaper if we are actually running low on
memory.

Bug: 28769940
Change-Id: I8aa27d081bbcc2eff553e9420b2b9b0920f3781f

packages/SystemUI/src/com/android/systemui/ImageWallpaper.java

@@ -166,8 +166,9 @@ public class ImageWallpaper extends WallpaperService {
         }
 
         public void trimMemory(int level) {
-            if (level >= ComponentCallbacks2.TRIM_MEMORY_RUNNING_LOW &&
-                    mBackground != null) {
+            if (level >= ComponentCallbacks2.TRIM_MEMORY_RUNNING_LOW
+                    && level <= ComponentCallbacks2.TRIM_MEMORY_RUNNING_CRITICAL
+                    && mBackground != null) {
                 if (DEBUG) {
                     Log.d(TAG, "trimMemory");
                 }