Clean redundant code in Plugin and JarPlugin

Below code in JarPlugin are same with that in Plugin,
can not find the reason why we need keep 2 copies of them.
Clean them to make the code dry and easy to read.
-function isModified() and variable 'snapshot'
-isDisabled() and variable 'disabled'
-class CacheKey
-function getApiType()

Change-Id: Id144a8a1c06320d506e660c1e5ca8c6cc7b36b49
1 file changed