Use  getByteCount in HONEYCOMB_MR1 or higher API
Trim memory cache when the device is running in low-memory
Use 'evictAll' to clear the cache, instead of resizing it.
Add a null check on mBitmapCache. It should never be 'null' but better to check it.
Add 'targetApi' annotation to prevent lint warning on PushNotificationBackendMonitor
Merge pull request #427 from wordpress-mobile/feature/425-free-memory-cache

Feature/425 free memory cache when the device running on low-memory