Build: Move preopt classpath to private variable

Move DEX2OAT_CLASS_LOADER_CONTEXT to
PRIVATE_DEX2OAT_CLASS_LOADER_CONTEXT in preparation for actual
classpaths.

Bug: 65256727
Test: m
Test: oatdump --oat-file=$OUT/system/framework/oat/arm64/wifi-service.odex --header-only
Change-Id: I5bcfebcd3be1a187954f550dbe1d3aa1d5099052