Address several lint warnings in soloader
Address several lint warnings in soloader

Summary:
This diff addresses several lint warnings:
* 'for' loop replaceable with 'foreach'
*  Non-atomic operation on volatile field 'sSoSourcesVersion'
* Return value of SoLoader.class.getClassLoader() is Nullable
* Use StringBuilder to replace "+" string concatenation

Reviewed By: danjin250

Differential Revision: D20309562

fbshipit-source-id: 7af131f6265dacb34a93609a431623e3a65f7115

facebook/SoLoader,facebook/SoLoader,facebook/SoLoader