Make TestPlatformBootclasspath_Fragments more realistic

Move the bootclasspath_fragment to be part of an apex to match how they
are expected to be used. Also adds the bar library to the boot jars
config as every module in a bootclasspath_fragment that is referenced
from a platform_bootclasspath module is expected to be in the config.

Bug: 179354495
Test: m nothing
Change-Id: I6f826dce9c006d630ee31d4171fe6fb458cc9442