Split functions for compiling boot jars to allow future code reuse.

This CL adds no functional changes, it only splits a few functions into
parts that will be reused in a follow-up CL.

Bug: 178467404
Test: lunch aosp_cf_x86_64_phone-userdebug && m
Change-Id: I2bcb3c03d51175130da29fdfaf6dd747e3aea964