Append platform classpath proto configs with missing apex jars.

Any apex classpath fragment that doesn't generate its own classpaths
proto, must still propagate it's boot jars for the platform classpath
fragment to include for complete CLASSPATH variables on device.

Bug: 191127295
Test: atest CtsClasspathsTestCases derive_classpath_test
Change-Id: I93687f69006741fcd66eb6e04891a4b4bbcc3b47