Use absolute paths in classpaths.proto.

The intention before was to use relative paths to a partition where a
config is defined. However, jars in /system_ext partition are planned to
be declared in /system's classpaths.proto config.

Bug: 180105615
Test: derive_classpath_test, CtsClasspathsTestCases
Change-Id: Icc3e1a903c34187cfcd67a3ae7bc3dd746445c03