[TIMOB-13436]Purging cached image if the underlaying local file changed.
[TIMOB-13436]-setting lastModified value within initialization.
[TIMOB-13436]minor ImageLoader update
[TIMOB-13436]Making image loader readonly.
Merge pull request #4180 from srahim/timob-13436

[TIMOB-13436] Refresh cache for imageloader if the underlaying local file changed.