init: add support of multiple class names

Add support of multiple class names in service, so that related services
can be grouped together. By doing this, we can start/stop some services
for special purpose. For example, early zygote, early boot animation
and etc.

Bug: 36535312
Test: marlin boots with defined classes
Change-Id: Ifeaaf034fd836816e24f3775bece53ea83faada6