Use another method to get ImageView in NavBar

All written in the extended commit message.
Should also solve this issue https://github.com/MohammadAG/Xposed-Tinted-Status-Bar/issues/16 

Like I said in this previous issue, https://github.com/MohammadAG/Xposed-Tinted-Status-Bar/issues/13 , this will be slower (about the same speed as findViewById), thus I added another ArrayList to cache the views. After caching the views, it shouldn't impact performance.